Not sure I agree with the idea of a general store though. Technically it's still a military ship (broadly speaking) and with the absence of money I imagine most material requirements would go through the Ship's Quartermaster as would any "snail-mail".
I think in TNG we saw something vaguely resembling a store on the E-D but that was basically a huge replicator for making giant bunnies. For "shopping" purposes I imagine it's usually more a case of the crew buying/trading things on shore leave than the other way around.
Military ships, especially those of the USN, have small stores for crew to purchase things such as candy, magazines, soap, etc. They are mini-marts/exchanges. When my father was an officer on a destroyer, I had a chance to see one. So, I don't think it would be out of the ordinary on a Starfleet vessel. Also, TOS did have use of the credit system as noted in "The Trouble with Tribbles" when Uhura took Chekov to do a "little shopping."
Oh sure, I know in the real world ships and bases have a shop or a Naffi for anyone to get what they need (I happen to live in an Army camp
) I just wonder if the term shop or store would be entirely appropriate to the kind of world GR was going for. As I understand it "Credits" are more of a resource allocation system than a form of currency, so if someone on board wanted say for example new clothes for off duty, an old fashioned time piece for their quarters or their own fencing foil to practice with (other than the standard issue utilities) they'd expend X amount of credits in the QM's department getting whatever it is they want synthesised (presumably with the same machine that can replicate Nazi uniforms and the like.) Food and beverages, of course is already taken care of with the slots, so no real need for a separate snack shop.
Perhaps I'm splitting hairs, but the idea of commerce as we're used to it (with stock, profit margins and the like) aboard the Enterprise just doesn't sit right with me...not that it makes the slightest difference to the deck plans other than what you happen to label the room that contains the non-organic synthesiser.