NO GRACIAS! explain to me

Rob, your scolding tone w/Carol is misplaced. She never said Mexican vendors should know what Americans are used to. What she meant, I think, is that if they did understand, then they'd know why they sometimes get the curt response they do.
The same might apply if tables were turned. Imagine a Mexican tourist visiting here in Minneapolis. At a restaurant he finds it rude and pushy that the waiter brings him the check as soon as he's finished his meal. The waiter (or owner) doesn't know that Mexicans are used to, shall we say, not being rushed to pay the bill, and wonder why the diner seems so upset. If they understood the cultural difference, they'd understand and maybe even try to handle things the way the customer is used to.
Seems to me it should work both ways. Why should Americans be the only ones expected to understand cultural differences?

One Man's Wonder