LB – I would only do it in a Frum store. To be perfectly honest, I think I only did it once (I don’t have a car, so it’s usually not possible), and that one time, not only was it a Frum store, but it was a Gemach store (everything costs around $3.00; some is new and some is hand-me-downs).
In any case, Frum people usually trust Frum people, and even not-Frum Jews often trust Frum people. I remember being in a not-Frum sock store when I was in a high school, and when I got to the counter, I realized I needed to ask my mother (who was in the car) something before I paid, so I asked the guy if I could go out to the car. He let me take the socks with me, saying something like, “of course I would trust a nice Jewish girl.”
And in Eretz Yisrael, it happened more than once that I didn’t have enough money, and the guy told me I could pay him back another time. Once, it was in Machane Yehuda! And another time in Angel’s.
Jews tend to trust other Jews, Boruch Hashem!