Your perspective is not wrong .... for 1999. With the gig economy and places like square, intuit, etc we have fixed fee structure vs variable. In the "old days" you had to sign up with MC, Visa, Amex, Diners, etc all separately. You might get a clearing house processor like Leaders or similar that would handle it all for you (for a fee), but you still had individual accounts with every possible processor. Some had monthly fees. some had minimums. some charged fixed fee per swipe plus %. some charged a % only. The % might be fixed or it might be variable based on whether the card was basic, premium or rewards. CC processing was highly variable and expensive to manage and impossible to predict.
Then the gig economy came along. Square charges no monthly and no swipe fee, just a fixed % per swipe and a higher fixed % per key in (internet, no card, etc). It does not matter if it is a platinum Amex or a debit card, they charge the same fee. They make a lot of money on debit swipes and can lose money on the super premium swipes. Over all they make a good penny for their processing services.
Small business are better aligned with the Squares than the old school processors. Big box, restaurants, other high volume places do better on old school processors.
In the gig economy your choices are VERY limited as a gun shop. You can find plenty of old school processors that will take your business, but most of the new ones wont touch you. Paypal and Square being two of the biggest think you are selling children. I use Intuit who is fine as long as I swipe but said they would cancel my account if I ever keyed in (no selling guns on the internet they say).
So unless you know who the shop is using to process payments, you don't know their fee structure. Yes, traditional debit is same as cash, but not if you use a Gig processor. The best I can do is echeck where it is only 1% fee instead of close to 3%. Debit is still the same as credit.
Margins are not flexible enough to price everything as if it was being paid for with credit. It is not an accident that the Deli Ticket Emporium in Woburn publishes cash and non-cash prices.
My perspective is from an employee of the largest US Payment processor there is. I'm the guy writing some of the code the handles payments.
Nothing you have said is incorrect and I don't believe anything I said was incorrect. We just have different viewpoints into the system. Your viewpoint is from a small merchant perspective while mine is from a payment processor who won't touch you if you are aren't doing a million a year or more.