Square API

A unified commerce platform providing REST APIs for payments, inventory, and customer management across physical and digital storefronts.

Square API connects your software to a complete financial stack: use the Payments API to process credit cards (PCI compliance included) or the Terminal API to sync with hardware like the Square Terminal. It handles more than just cash flow; the Catalog API manages complex inventory while the Customers API tracks loyalty and purchase history. Developers build custom POS systems and e-commerce checkouts using SDKs for languages like Python, Ruby, and Node.js. This ecosystem (supporting millions of active sellers) turns raw code into a professional business engine.

https://developer.squareup.com