Okay, @lukebuxton - i’m a little behind, but I did not forget you!
There are two solutions for this in Wix.
1: You can use the inventory management of multiorders, it’s a third party app - Here is the link to the app market.
2: You can also accomplish this using Corvid - The team mentioned that there will be documentation coming along soon to outline the process.
This should get you going!