it depends partly on the state, some states are rolling back the law that says you can not, others have not. but ... this is the most current I can find
https://www.finder.com/review-guide-to-credit-card-surcharges

Since you can not, in any state, surcharge for a debit or prepaid card, there is no way to issue a surcharge for online sales at all - you do not know the type of card being used.

And in QB an invoice is final, there is no way to issue an invoice that says this much if you pay by cash/eft/elect check, or this much if using a credit card.

I just don't see the issue myself, I use square, so the only time I deal with a merchant processor fee is when I make a sale - no monthly dues or anything - so to me it is a cost of doing business, less than 3% of the sale in actuality.  Now I can see a sign saying something like 4% discount if paid with cash/check.  But then again now you have to deal with bounced checks , and if that happens it is a much higher fee.