I agree with all of the above. And also, for the love of all that is holy, when you offer a sandbox and test data, give me a way to deliberately trigger error

I agree with all of the above. And also, for the love of all that is holy, when you offer a sandbox and test data, give me a way to deliberately trigger error responses. It's all well and good to give me an is_test parameter on your payment API, but if using it causes you to blindly approve all payments, I can't test all of the code paths dealing with all of the possible failure modes(true story).