For example, if 90% use Credit card, go with a simpler and cheaper payment provider and implement a mechanism that switches between them based on some frontend configuration.
I've worked for a company that they've essentially done that.
I know you might then need to handle having two places where you have invoices etc, but for the amount you are transactioning, it might be the right way to go.
This isn't important only to reduce how much you pay, but your lock-in and dependency on them, otherwise you have no leverage on those negotiations.
Your pain, their profit!