You're a guest when you're off the clock. My store always said shopping is fine on a 30, but they do find it acceptable to buy food for my meal during my 15. I asked that specific question once because otherwise I'd spend 15 of my 30 walking around undecided.
But most of the time, if you're in the building, I'd think you could just buy it then. I've had stuff held when lines would've prevented me from getting back from my 30 on time-ish, but I was prepared to pay then. And I wasn't buying any limited items or anything so no one cared.