I’m better at restaurants than with cashiers, but I still hate it. My sister’s probably even worse than me, so if we’re out together, whoever is more comfortable, either with that particular store/restaurant or in general is the one who ends up doing the ordering/general dealing with people.
Knowing exactly what I want helps some, but even still I end up reading off the menu what it is because my mind just draws a blank and a lot of the time I just avoid having to talk anymore than necessary. If they mess up, I usually need someone else to actually talk to waiters/cashiers/managers, because I just won’t. I’ll go hungry instead of talking to them, and I hate it, but I don’t really know of any way to fix it.