Every gas station is surcharging. You can word it as a cash discount and case is closed.
Even for debit cards? And question is not if stores are doing it, but whether it's legal (regardless of the wording). And what if a merchant clearly states that it is a processing fee?
(2)Limitation on restrictions on offering discounts for use of a form of payment(A)In generalA payment card network shall not, directly or through any agent, processor, or licensed member of the network, by contract, requirement, condition, penalty, or otherwise, inhibit the ability of any person to provide a discount or in-kind incentive for payment by the use of cash, checks, debit cards, or credit cards to the extent that—(i)in the case of a discount or in-kind incentive for payment by the use of debit cards, the discount or in-kind incentive does not differentiate on the basis of the issuer or the payment card network;(ii)in the case of a discount or in-kind incentive for payment by the use of credit cards, the discount or in-kind incentive does not differentiate on the basis of the issuer or the payment card network; and(iii)to the extent required by Federal law and applicable State law, such discount or in-kind incentive is offered to all prospective buyers and disclosed clearly and conspicuously.(B)Lawful discountsFor purposes of this paragraph, the network may not penalize any person for the providing of a discount that is in compliance with Federal law and applicable State law.
https://www.law.cornell.edu/uscode/text/15/1693o-2Yeah, the restriction on CC surcharges becomes a bit of a joke due to this.