Being more than stupid about conditions in Afghanistan, do they have PX's, for instance, at FOBs?
Yes, at most of them. If it's a small FOB, they might only have a single-wide trailer or something, but you can usually get the basic necessities at those.
I've only spent a few days in Afghanistan (and that was enough), but AAFES has a pretty big presence there. They not only run the PXs, but they manage the Pizza Hut, Subway, etc. restaurants that tend to be on the bigger bases.