Shared sheets feel flexible until two people edit the same size column. Suddenly “available” means three different things, and the person packing the box is the last to find out.
One catalog for the shop is boring on purpose. SKUs are unique inside that shop, reserved units are visible next to on-hand units, and retiring a product keeps history without cluttering what the inbox is allowed to promise.
That discipline is what lets any reply (typed, automated, or AI-drafted) answer without guessing. In DigiYar the keyword reply, the checkout link, and the packing list all read the same row.
Put this into practice
Create shop account