An extremely easy popular piece of legislation would be to ban these sort of junk fees.

Here in Chicago it's extremely annoying because you can pay physically at the meter with a credit card and not get hit with these junk fees, but if you use their app, you have to pay the fee, and extending the time hits you with the fee again. The app is a piece of garbage that hasn't been meaningfully updated in years and it's not like maintaining the physical meter is free either. Plus when you use the app they require you to load a minimum $20 balance into it so all the Abu Dhabi princes who own our public infrastructure get to make interest on that hostage money. When I street park I make it a point to pay at the meter with my most expensive Amex.

A better solution would be to ban using phone apps to pay for parking.

there must be a law to cap these fees at 4-5% of the service. This is roughly 2x of the cost to process credit card payment

Might cost them more paying in change to be honest. Someone's earning a good city pension from taking those quarters out of the machine.