If she really is a relapsed alcoholic, maybe she got off the highway to find a place that sold alcohol? It would explain going to different stores - and we only know the ones she used her credit cards/owners stepped forward. Even though the diner waitress said SS didn't order a drink, maybe she went in, ordered the food, and then realized they didn't serve drinks - so she left? Then tried the other markets?
Or maybe the stress of getting lost in Olympia with a hungry kid on the way to a reunion with a family member she hadn't spoken with in a long time was enough to make her want a drink?
I do think the waitress said they serve alcohol at the diner. I haven't been in since they changed the name a few years ago, I don't remember if they do.