So a project would use omnipay and then select what processors they want their clients to be able to use. Clients bring their own API keys or OAuth.
I believe it's in PHP and Ruby.
Unfortunately none of them worked for me, because PayPal (the only one usable in my country) has been unusable. One of the big reasons is because the only bank that supports them in my country can't create a usable online profile.
But even with PayPal itself, they think I haven't confirmed my email address when I have. Although I stopped getting those emails, so perhaps the situation resolved itself.
I'm going with Lemon Squeezy now.
The lesson I have learnt in this ordeal is that you need to confirm, as much as possible, that the payment provider is usable for you. Are you approved? Are payouts accepted in your country? Tick every single box you can before you write a single line of code for a specific payment provider.