BTCPay Server is an Open Source payment processor, written in C#, that conforms to the invoice API of Bitpay. This allows easy migration of your code base to your own, self-hosted payment processor.
This solution is for you if:
You are currently using Bitpay as a payment processor but are worried about their commitment to Bitcoin in the future
You want to be in control of your own funds
Bitpay compliance team decided to reject your application
You want lower fees (we support Segwit)
You want to become a payment processor yourself and offer a BTCPay hosted solution to merchants
You want a way to support currencies other than those offered by Bitpay