I'm trying to understand exactly what you are asking for.
Is a subscription box a collection of products? Are there different variations of boxes? Are boxes unique (user chooses what goes in it?)
Let's say you choose what items go in a box. I want my box to have 2 blue bandanas. Let's say you only have 1 location (Location A). Your location has 100 blue bandanas in stock. After confirming this subscription, Location A blue bandana stock should automatically update from 100 to 98. Therefore, if another customer attempts to purchase this box, my existing theme will automatically flag an error of some sort, or disable the option, when and if the stock gets down to 0 (or has less than the order amount).
Did I understand that correctly?
Gotcha. Existing subscription apps don't update the inventory in real time.
I'm not sure if any other apps will do this for you. There does seem to be quite a few in this space. It's an aggressive one.
I'd be potentially open to making an app that does just that. It does strike me as a pretty ambitious project. Could be fun. If you feel definitely that no app on the store already supports what you are looking for; feel free to email me: firstname.lastname@example.org