You can upgrade the item shop so it sells more items. You do so by spending bells at the item shop and doing a few other things. Only items that can be sold out count toward your total. In other words, stationery, wrapping paper, and melody cards don't count toward the bell total. Fortune cookies are counted as 500 bells.
Upgrade to the Convenience Store (T&T Mart)
To upgrade to the convenience store (T&T Mart), you must have a house rather than a tent. Upgrade from the tent by giving 10,000 bells to Tom Nook. The convenience store upgrade occurs after 12,000 bells have been spent in the shop (all players on the game card can contribute) or you have bought at least 15 items. It must be at least 10 days since you created the town.
Upgrade to the Supermarket (Super T&T)
The supermarket is called Super T&T. The supermarket upgrade occurs after you have spent 25,000 bells in the shop. It must be at least 10 days since the convenience store opened. It must be at least 10 days since the garden center opened.
Upgrade to the Home Center (T.I.Y)
The upgrade to the home center (T.I.Y) occurs after you have spent 50,000 bells in the shop. It must be at least 21 days since you built the supermarket.
Upgrade to the Department Store (T&T Emporium)
The upgrade to the department store (T&T Emporium) occurs after you have spent 100,000 bells in the shop. It must be at least 30 days since you built the home center. You also have to pass Gracies fashion check at least four times before the emporium will open.