Earlier quoted context omitted.
> A 5% flat fee is too much for me though, if it had a cap then it'd be different. It can't have a cap if payment gateway processing fees are percentage-based.
Of course you can still have a cap. "A payment processing fee of 0.5% + 0.20c is collected on the total donation. We take an additional fee of 4.5%, up to a maximum of $10" There, done.
Stripe's payment processing fee is 2.9% + 30¢ in the US.