Something like a pick-up window would work.

> do you show your phone to the person and hope they have good organization of orders pending pickup?

In my opinion, if the food truck has implemented this Square software, they might keep "organization of orders pending pickup" in mind. Showing your phone to the person should be enough, (with some sort of simple identification on the phone message).

I don't know if Square Pickup does this but I'm just arguing against your doubt that it wouldn't work.