Actually I find BTCPayServer very interesting but then you really have to implement everything by yourself including the exchange part to convert in fiat and transfer to the merchant's bank account? How would you do that?
I assumed that you want to keep the crypto. If fiat is involved, then you are better off with Coinbase or BitPay but as I said, there will be fees.
Why you think coinbase should be better than Bitpay?
I basically need a way to deduct a fixed amount (let's say 1%) from the merchant wallet to another wallet in the most secure way. Possibly within the same platform without storing the API keys outside of id. Any Idea?