Someone with the right skill set should make it their goal to live all these companies like Mises.org off of shitty "crypto" payment processors and onto BTCPayServer via something like Voltage.
Discussion
What are some BTC payment processors? Any turn key options?
Yeah IbexPay is probably the easiest one that I know of. But I'm not sure how well it integrates with websites. Otherwise I think you can use voltage to run a node in theclodud and run a BTCPayserver. Allegedly they make it pretty easy, but I haven't tried it.