I know it's not part of the principle, but is there anything in the rules that says you can't charge an additional fee to those signing up using in app payments

I know it's not part of the principle, but is there anything in the rules that says you can't charge an additional fee to those signing up using in app payments? If this is the hill Apple chooses to die on, every Apple customer just has to pay an additional 30ish% for a service, but of course you can always sign up online for the special sale price.