Perhaps this one…
https://www.wix.com/corvid/reference/wix-billing-backend/invoices-obj/addpayment
or this one…
https://www.wix.com/corvid/reference/wix-pay#top
or this?
https://www.wix.com/corvid/reference/wix-pay-backend#top
Already have seen this?
or this…
https://www.youtube.com/watch?v=OPzzmEhY4dw
Let me know if you found the right one
And here one more…
https://support.wix.com/en/article/corvid-tutorial-processing-payments