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The question is: Can you interoperate without using the proprietary app? Facebook also builds on the internet and the web, but you can not interoperate with Facebook without becoming a Facebook user. And "interoperate" means that you can use an alternative while the other party does not (i.e., you are not dependent on the other party's cooperation in order to be able to use an alternative).

But also, in any case it's still a bad idea because the payment provider has the ability to just disobey your payment orders, and regulation doesn't help with that but rather potentially makes it even more likely "for security reasons". Payment providers use secret methods to decide whether your payment order is considered a security risk and then will prevent you from spending your own money based on their secret decision making process, or at least make you jump through unpredictable hoops before they allow you to spend your own money. That's a major security risk for you, in addition to all the security risks due to the well-known terrible IT security practices of banks as well as/in combination with the privacy risks of leaving a digital trail of your locations and purchases.