I'm running into the same issue - Squarespace seemed to advertise that they can sync inventory but this doesn't seem to actually be the case. I've got an issue where my inventory is in Squarespace and I need to offer employees a way to use the POS app. There isn't a way in squarespace to give permission just to use the POS app (and I don't even know what admin permission I'd give them to do it) - and if they use the Square app instead it wont keep track of our inventory.
GRRRR!!!!
