I'm a solo dev building a marketplace, and payment integration always felt like a black box. Then I came across this online payment API guide that breaks it down step by step: setting up credentials, making your first API call, handling webhooks, and even dealing with error codes. The sandbox testing section was a lifesaver — I had a test transaction running in under an hour. Also explains tokenization and 3‑DS in plain language. If you're a developer adding payments to your app, this is the clearest resource I've seen.