Fee Assessed by Retailer for Accepting Credit and Debit Card Payments

It depends on where you live. Where I live, the fee is already included, but some retailers (and my dentist, for instance) will give you a discount if you pay cash. I don't think it's illegal to add the fee, but usually there's a sign at the till offering the discount for cash.

I see gas stations do this regularly..they show the price if you use credit and the price you pay to use cash.

I would imagine the only legal side is in regard to making it clear there is a fee or difference in price.

I'm pretty sure you're right.