In general, the principle of the idea is the same, whether it is for trade stores or donations, which is the fear of fluctuating, so they are exchanged directly for fiat. you can do it with open source payment gateway ---> https://btcpayserver.org/ & Convert to fiat via https://www.blockonomics.co/views/payment_forwarding.html
I think this is the right guide for blocknomics to directly convert them instantly into fiat with 3rd party service.
The BTC will redirect to Uphold wallet and convert it into fiat or any stable coin.
The only problem on direct convert to fiat USD/EUR it needs KYC and you must be verified who have a connected US bank account. So if you are not living on US you won't be able to use their instant exchange service(BTC to fiat).