Build a Paddle Checkout in Next.js

When to use this skill

Use this skill when adding a hosted Paddle checkout to a Next.js (App Router) app, either as an overlay (modal-style) or inline (embedded within your page). It covers initialization, opening checkout with line items, listening to events, customer pre-fill, the post-checkout redirect, and the throttled updateItems pattern for cart-like UIs.

This skill is client-side only. Pair it with:

• webhooks to receive transaction.completed / subscription.created events on your server.
• subscription-sync to mirror Paddle subscription state into your database.
• sandbox-testing to test the integration end-to-end in the Paddle sandbox.

Prerequisites

You need:

• A Paddle account, with the sandbox environment active during development.
  • Sign up for sandbox: https://sandbox-login.paddle.com/signup
  • Sign up for live: https://vendors.paddle.com/
• At least one product and price set up. Use catalog-setup if you haven't yet — note the price ID (looks like pri_01h...) for use below.
• Your domain approved for checkout. In sandbox, checkout opens on any page, including localhost, and domains you add are approved automatically. In live, the user submits domains under Paddle > My account > Settings > Website approval — there's no API to submit one. If a Paddle MCP server is available, check approval with client.checkoutDomains.list() (GET /checkout-domains): each domain has a status, and only approved domains can run checkout.
• Your default payment link set. Use a page on your own domain, not localhost. In live, its host must be a domain approved for the account, or that domain with www.. Sandbox doesn't need an approved domain. If a sandbox update returns checkout_url_domain_not_approved, add the domain under Paddle > My account > Settings > Website approval, where sandbox approves it straight away, and try again. If a Paddle MCP server is available, read it with client.settings.checkoutUrl.get() (GET /settings/checkout-url) — default_url is null until set. After asking the user, set it with client.settings.checkoutUrl.update({ default_url: "https://..." }) (PATCH /settings/checkout-url, needs the checkout_settings.write permission). If that's unavailable or returns forbidden, ask the user to set it under Paddle > Checkout > Checkout configuration > Default payment link.
• Two environment variables. Create the client-side token under Paddle > My account > Settings > Authentication, on the Client-side tokens tab:

NEXT_PUBLIC_PADDLE_CLIENT_TOKEN=test_...   # client-side token, safe to expose
NEXT_PUBLIC_PADDLE_ENV=sandbox             # or "production"

If a Paddle MCP server is available to you, call client.clientTokens.create({ name: "Frontend dev token" }) inside an execute to provision the token programmatically rather than asking the user to copy it from the dashboard. Note clientTokens is camelCase. Creating a token needs the client_token.write permission.

The Paddle MCP exposes three tools per server (search, execute, report_missing_tool). Workflow: call search to confirm the exact method name and parameter shapes, then call execute with an async function that calls client.<resource>.<operation>(...). Method paths are camelCase (client.clientTokens.create, client.pricingPreview.preview). Body params and response fields are snake_case (tax_category, product_id, unit_price, currency_code). List methods return { pagination: { hasMore, estimatedTotal }, <resource>: [...] }, not data. The array's key is the resource's API path segment, so it's kebab-case where the method path is camelCase: (await client.products.list()).products, (await client.clientTokens.list())["client-tokens"]. Page with { after: "<last_id>" } while pagination.hasMore is true, not .next(). Lists that aren't paginated, like client.currencies.list(), return a plain array. Single-entity methods return the entity. Chain multi-step workflows inside one execute; variables don't persist between calls. Hard caps: 50 API calls per execute, 30s timeout, and 32,000 characters of code.

Install the client library:

npm install @paddle/paddle-js

The inline example below throttles updateItems with lodash.throttle. If you use that pattern, install it and its types too:

npm install lodash.throttle
npm install -D @types/lodash.throttle

The Node SDK (@paddle/paddle-node-sdk) is for server-side work — you do not need it for checkout.

Choose your checkout style

Both use the same Paddle.Checkout.open() call — the only difference is the displayMode setting and where it renders. Default to overlay unless you need branded inline. Overlay works without any layout changes; inline requires a target element.

Choose your checkout variant

Multi-page is the default. One-page is recommended for most use cases.

Express (variant: "express") has extra rules:

• Inline only (displayMode: "inline").
• Checkout won't open if you combine it with an authenticated customer (customerAuthToken), upsell, allowedPaymentMethods, or a business on the transaction.
• showAddDiscounts, allowDiscountRemoval, showAddTaxId, and allowLogout are ignored.
• Other payment methods show alongside the wallets unless you set showNonExpressPaymentMethods: false (defaults to true).
• The types need @paddle/paddle-js 1.6.4+ for express, and 1.6.5+ for showNonExpressPaymentMethods.

Overlay checkout — the minimum viable integration

This is the fastest path: a button that opens checkout for one price. Recommended.

// app/buy/page.tsx
"use client";

import { initializePaddle, type Paddle } from "@paddle/paddle-js";
import { useEffect, useState } from "react";

export default function BuyPage() {
  const [paddle, setPaddle] = useState<Paddle | null>(null);

  useEffect(() => {
    if (
      !process.env.NEXT_PUBLIC_PADDLE_CLIENT_TOKEN ||
      !process.env.NEXT_PUBLIC_PADDLE_ENV
    ) {
      return;
    }
    initializePaddle({
      token: process.env.NEXT_PUBLIC_PADDLE_CLIENT_TOKEN,
      environment: process.env.NEXT_PUBLIC_PADDLE_ENV as
        | "sandbox"
        | "production",
    }).then((p) => p && setPaddle(p));
  }, []);

  function openCheckout() {
    paddle?.Checkout.open({
      items: [{ priceId: "pri_01h...", quantity: 1 }],
      settings: {
        variant: "one-page", // or "multi-page"
      },
    });
  }

  return (
    <button onClick={openCheckout} disabled={!paddle}>
      Buy now
    </button>
  );
}

That's it for overlay — no displayMode setting needed (overlay is the default). The user is shown the modal, completes payment, and Paddle handles the success page.

Inline checkout with full event handling

Inline checkout is what you want when you need to render checkout next to other UI (e.g. a price summary, branded layout, custom success state). The full pattern:

// app/checkout/[priceId]/checkout-contents.tsx
"use client";

import {
  type Environments,
  initializePaddle,
  type Paddle,
} from "@paddle/paddle-js";
import type { CheckoutEventsData } from "@paddle/paddle-js/types/checkout/events";
import throttle from "lodash.throttle";
import { useParams } from "next/navigation";
import { useCallback, useEffect, useState } from "react";

interface Props {
  userEmail?: string;
}

export function CheckoutContents({ userEmail }: Props) {
  const { priceId } = useParams<{ priceId: string }>();
  const [quantity, setQuantity] = useState(1);
  const [paddle, setPaddle] = useState<Paddle | null>(null);
  const [checkoutData, setCheckoutData] = useState<CheckoutEventsData | null>(
    null,
  );

  // Throttle updateItems to avoid hammering Paddle on rapid quantity changes.
  const updateItems = useCallback(
    throttle((paddle: Paddle, priceId: string, quantity: number) => {
      paddle.Checkout.updateItems([{ priceId, quantity }]);
    }, 1000),
    [],
  );

  useEffect(() => {
    if (paddle?.Initialized) return;
    if (!process.env.NEXT_PUBLIC_PADDLE_CLIENT_TOKEN) return;

    initializePaddle({
      token: process.env.NEXT_PUBLIC_PADDLE_CLIENT_TOKEN,
      environment: process.env.NEXT_PUBLIC_PADDLE_ENV as Environments,
      eventCallback: (event) => {
        if (event.name && event.data) {
          // Every data event, checkout.loaded included, carries the latest totals.
          setCheckoutData(event.data);
        } else if (event.type === "checkout.warning") {
          // Warnings have no `name`: checkout opened, but some values were dropped.
          console.warn("[Paddle]", event.detail);
        } else if (event.name) {
          // checkout.error / checkout.payment.error: flat { name, type, code, detail, documentation_url }.
          console.error("[Paddle]", event.name, event.code, event.detail);
        }
      },
      checkout: {
        settings: {
          variant: "one-page",
          displayMode: "inline",
          theme: "dark",
          allowLogout: !userEmail,
          frameTarget: "paddle-checkout-frame",
          frameInitialHeight: 450,
          frameStyle:
            "width: 100%; background-color: transparent; border: none",
          successUrl: `${window.location.origin}/checkout/success`, // must be absolute
        },
      },
    }).then((p) => {
      if (p && priceId) {
        setPaddle(p);
        p.Checkout.open({
          ...(userEmail && { customer: { email: userEmail } }),
          items: [{ priceId, quantity }],
        });
      }
    });
  }, [paddle?.Initialized, priceId, userEmail]);

  useEffect(() => {
    if (paddle?.Initialized && priceId) {
      updateItems(paddle, priceId, quantity);
    }
  }, [paddle, priceId, quantity, updateItems]);

  return (
    <div>
      <PriceSection
        checkoutData={checkoutData}
        quantity={quantity}
        onQuantityChange={setQuantity}
      />
      {/* The class name here MUST match `frameTarget` above. */}
      <div className="paddle-checkout-frame" />
    </div>
  );
}

Three things to notice:

1. frameTarget is a CSS class name (no leading dot). Whatever you set here, you must also render an element with that exact class. Paddle injects the iframe into it.
2. eventCallback fires for every event — use the event.name to discriminate (checkout.loaded, checkout.items.updated, checkout.completed, checkout.payment.failed, etc.). The example keeps the latest event data so a sibling component can show price totals. Taking it from every data event, checkout.loaded included, means the totals are right before the first update. Error and warning events have a different shape — see "Reading checkout state" below.
3. successUrl is where Paddle redirects after checkout completes. It must be an absolute http(s):// URL, so build it from window.location.origin. Checkout.open() throws on a relative URL, and a relative URL passed to initializePaddle is dropped: checkout emits checkout.warning and doesn't redirect. Make this a route that handles post-purchase UX (e.g. shows an order confirmation) — but do not rely on it for provisioning; the webhook is the source of truth.

Customer pre-fill

If your user is already authenticated, pass their email so they don't need to type it:

paddle.Checkout.open({
  customer: { email: "jane@example.com" },
  items: [{ priceId: "pri_01h...", quantity: 1 }],
});

To go further, pass an existing Paddle customer ID (created via the API or returned by a previous checkout):

paddle.Checkout.open({
  customer: { id: "ctm_01h..." },
  items: [{ priceId: "pri_01h...", quantity: 1 }],
});

Setting allowLogout: false in the checkout settings prevents the user from signing out of their pre-filled session — useful when you've already authenticated them.

Dynamic line items

To change quantity, swap a price, or add another item without closing the checkout, call Paddle.Checkout.updateItems(). It replaces the whole list — any item you leave out is removed:

paddle.Checkout.updateItems([
  { priceId: "pri_01h...", quantity: 3 },
  { priceId: "pri_02h...", quantity: 1 },
]);

• Recurring items must share the same billing cycle and trial period.
• An empty list, or any item with quantity below 1, is ignored. To remove an item, send the list without it.
• Check the list against these rules before you call it.

Always throttle these calls (1 second is a good default — see the lodash.throttle example above). Paddle re-renders the checkout on each call, and unthrottled updates produce a flicker and rate-limit risk.

Reading checkout state

Hook into the eventCallback to drive your own UI (running totals, line item breakdowns, applied discounts). The most useful events:

Error events (checkout.error, checkout.payment.error) have no data. Their name, type, code, detail, and documentation_url are top-level fields on the event (since @paddle/paddle-js 1.6.1). checkout.warning has no name at all, so match it with event.type === "checkout.warning".

Full list at developer.paddle.com/paddle-js/events.

Post-checkout: redirect vs webhook

When checkout completes, two things happen in parallel:

1. Checkout fires checkout.completed and sends the user's browser to your successUrl (or your custom event handler). For some payment methods the payment is still processing at this point.
2. Paddle fires a transaction.completed (and possibly subscription.created) webhook to your server.

Provisioning belongs in the webhook, not the redirect or checkout.completed. The redirect is for UX (showing "Thanks for your order"); the webhook is the durable, retried, signed event you can trust. See webhooks for setup.

Common pitfalls

• Something went wrong message — typically means there's no default payment link, or, in live, the domain isn't approved for checkout. With a Paddle MCP server, check both: client.settings.checkoutUrl.get() (GET /settings/checkout-url) should return a default_url, and in live client.checkoutDomains.list() (GET /checkout-domains) should list that URL's domain as approved. Without MCP, ask the user to check Paddle > Checkout > Checkout configuration and Paddle > My account > Settings > Website approval.
• Paddle is not defined — you forgot to await initializePaddle() before calling Paddle.Checkout.open(). The promise resolves with the Paddle object; only then can you open checkout.
• Checkout doesn't render inline — your frameTarget class name doesn't match a rendered element, or the element isn't in the DOM yet when Checkout.open() runs. Render the target first; open in a useEffect.
• Sandbox vs production drift — NEXT_PUBLIC_PADDLE_ENV controls which environment Paddle.js talks to. If your client token is for sandbox but you set NEXT_PUBLIC_PADDLE_ENV=production, checkout will fail to load. Tokens and prices are environment-scoped — sandbox pri_... IDs don't exist in production.
• Pasting price IDs into client code — fine for a quick prototype, but for a real app load price IDs from the server (after fetching the catalog) so you can swap them without a redeploy.
• Using the redirect or checkout.completed for provisioning — checkout.completed can fire while the payment is still processing, and users close tabs, lose connections, or block redirects. Webhooks are the source of truth.
• Relative successUrl — Checkout.open() throws, and one set through initializePaddle is dropped with only a checkout.warning to show for it. Use `${window.location.origin}/checkout/success`.
• Throttling updateItems — without throttling, rapid quantity changes flicker and may rate-limit. 1 second is a sensible default.
• Calling initializePaddle twice — the second call doesn't initialize again; it calls Paddle.Update() with the new options, so they replace the ones you set first. Guard with paddle?.Initialized (as in the example) or use a singleton pattern, and pass per-checkout settings to Paddle.Checkout.open({ settings }).

Verify the integration

1. Run npm run dev and navigate to your checkout page.
2. Confirm the checkout loads or the iframe renders and the price matches what you expect.
3. Use a Paddle sandbox test card — e.g. 4242 4242 4242 4242 with any future expiry and any 3-digit CVC.
4. Complete the purchase and confirm:
   • The browser lands on your successUrl.
   • The checkout.completed event fired (console.log from eventCallback).
   • In the Paddle dashboard (sandbox), the transaction shows under Transactions.
5. To test the webhook side, see sandbox-testing for the simulator.

Related docs

• Paddle.js overview
• Build an overlay checkout
• Build a branded inline checkout
• Checkout events reference
• Paddle.Checkout.open reference
• Default payment link & domain approval
• Reference implementation: paddle-nextjs-starter-kit — see src/components/checkout/checkout-contents.tsx.