Kind of strange, after reading this with a more clear thought. It still doesn't make
sense. If 20 bottles was on a shelf, (and for the sake of just having numbers) 10 people bought 2 bottles each. Technically no one should be able to purchase any more if those 20 bottles were gone (ie, in the cart). Now for the case of abandon carts, maybe some type of idle or timeout will help with putting those stock back on the shelf.
So an instance would be that someone had abandon a cart with 2 bottles, that would go back on the shelf. Now lets say someone else didnt' want 2 bottles, but one bottle. Now that would be 3 bottles total back on the shelf. So if someone was to go back to check the shelf and now will see something on stock will be able to add them to there cart.
If the case that someone was to try to grab more than what was on the shelf, maybe a a more real time inventory number on the side might help with that issue. Or a message saying that there is only so and so number of bottles able to purchase.
I'm not sure how the cart system PS is using works, but either way that works with them is how it will go. Just making some obvious observation.