I've used FastSpring for a long time, and FastSpring offers my customers many payment options including credit card, PayPal, Amazon Pay, bank transfer, etc. I found that Amazon Pay was consistently about 1% of my sales. (For comparison, direct use of credit card was about 65%, PayPal, about 30%)
I wanted Amazon Pay to be much more successful when it launched, but for some reason, at least in my case, it never was much used.
Can you use it to set up recurring payments when the customer doesn't have a credit card?
Can't answer that part of your question, I'm afraid.