If you try to equip anything from a store you get a message saying that you should pay for it first.

But it looks like reloading your ammo from the quick reload dialogue doesn't make this check.

e.g.

"The arrow (+0, 1d6) doesn't hit anything. Out of ammunition.
Reload with the bundle of 17 arrows (-1, 1d6) (unpaid, 68gp)?"

The shopkeeper won't let me out of the store with the arrows equipped, but he doesn't care if I fire them and they disappear afterwards.

It's easy to reproduce if you have a bow and Barnabas is selling ammo for it.