Yup. We had interesting experiences in Vietnam where everything is in cash, but the occasional merchant would accept Visa. In all cases, the merchant would apologetically inform us that they would have to add a 3% processing fee to our bill..

Exactly. Better yet: just increase the exchange rate for the customer to effectuate a discount. As a bitcoiner who probably has a good idea that the exchange rate has been hovering around 104K, imagine a coffee shop offering you 107K to spend your sats instead of dollars. You check your phone to confirm: hell ya, that's a good rate right now. If that's me, I'm choosing to spend sats; and more importantly, I'm making that my new go-to coffee shop.