Ok, so you say that the branded store would be aware of this and consent to it, right? Good, because people who copy other stores suck.

So what I think you want is a ‘marketplace’ type of store? You would have an API that you place from their store to yours, or vice versa (?) and sales sync. So when you sell something, they get the sale and ship it for for. I used to have our products on a few marketplaces and added their API’s to my store, so when they sold something on their store, I got the notification and shipped it. Soooo, I assume that you would arrange an API at your end, and enter it on the sub store, and when you sell something, they ship it for you. There are companies who do marketplace stores but they’re not cheap. Maybe you don’t need them…maybe it’s easy for a dev in upwork. Hope this helps?