The Selling Partner API (SP-API) is a REST API (an interface that other programs call over HTTP) from Amazon. It gives Amazon sellers and vendors, and the applications they authorize, programmatic access to their own business data: orders, shipments, payments, inventory, listings, prices, financial transactions and reports.
With it a developer builds applications that process orders and send shipping confirmations, track inventory across warehouses, create listings, update prices, and fetch financial transactions, tax information and invoices. Notifications send alerts when orders are placed or shipping status changes. The Data Kiosk API returns sales and traffic data. A developer registers once; each seller then grants access through an OAuth authorization workflow (a standard way to give an app limited access without sharing a password) from the Selling Partner Appstore or the developer's website. Amazon supplies tools to generate SDKs and a sandbox for testing.
Amazon offers the API without a fee: it had announced annual and usage fees for third-party developers, then decided not to move forward with them.