On the other hand, almost every merchant and waiter in Spain told me, when handing me the card terminal, to select "local currency" (decline the first swindle attempt) then "don't convert" (decline the second swindle attempt). There's obviously some required workflow where they must pass the terminal to the customer, but they are wise to the payment gateway's trick to extract additional value from the transaction. They don't want their customer bilked, or to take the reputational damage when the customer leaves an angry review.

So if your Mexican merchants "don't know" what their terminal says? Either you were their first foreigner, or they're useful idiots, or they know.