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PayPal Developer

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PayPal's official developer portal with API reference, SDKs, integration guides, and a sandbox for testing. Developers learn to add PayPal Checkout, process orders and refunds, handle webhooks, and test payment flows with sandbox accounts before going live.

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stripe.com

Stripe is a payments platform offering API-based online payment processing, subscriptions, and related financial tools for businesses. Its site provides developer documentation, API references, integration guides, checkout components, pricing, and tutorials to learn how to implement and optimize payment gateways.

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