Ask HN: Resources for hardening software handling payments?

I've recently started building out the API backend for a super early stage startup that will be handling credit card payments. I have some professional web development experience so I am definitely capable of diving in and getting started, but I've never written software that will be responsible for handling payments and I'm a little intimidated. A few things occur to me to incorporate into my design, such as:

1. Going through the OWASP top ten in detail before release 2. Ensuring that my endpoint that actually executes payment for an order is idempotent and atomic, to avoid duplicate charges and inconsistent state

But I'm looking for some resources that might help me identify things I don't know to look for. In addition I would love recommendations for resources on relational database design, so that I can try to model as many system invariants in SQL as possible. Thanks for any recommendations.