As opposed to my horrific experiences with PayPal, Swish, a Swedish (really smooth) payment processor has some really nice documentation. An example:

"The callback, in the happy case, will return an intermediate response with the status DEBITED."

And other nice things such as clear numbered lists describing user flows, with images for extra clarification. Also, they provide full lists of error responses and in many cases suggested way to proceed with these error cases.

And as the cherry on top, this is developed as a cooperation between a few Swedish banks. The banks, who are the most thick type of companies when it comes to IT, does it better than PayPal.