PayPal

1. Get started

   The REST API, sandbox accounts and first calls.

2. Authentication

   Getting OAuth 2.0 access tokens with client credentials.

3. Orders API

   Create, authorize and capture orders.

4. Checkout

   Standard checkout and Smart Payment Buttons.

5. Card payments

   Accepting cards with advanced card fields.

6. Subscriptions

   Subscriptions with plans, billing cycles and statuses.

7. Invoicing

   Creating and sending invoices.

8. Payouts

   Sending payouts to many recipients.

9. Webhooks

   Webhook events and signature verification.

10. Disputes

    Handling customer disputes and chargebacks.

11. Sandbox testing

    Sandbox accounts and testing.

12. Errors and idempotency

    Error responses and the PayPal-Request-Id idempotency header.

13. Platforms and marketplaces

    Multiparty payments for marketplaces and platforms.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.