Repeating / recurring purchase is something that you buy on a regular basis. For example you might be buying milk every week. If this is the case then you can make it a recurring expiry.
You can setup item expiry from "Edit Storage Details" for the item. Tap on the item row (anywhere on the right side of the row except the + or list icon) to bring up the edit screen. Go to the Expiry section and tap on row ""Expiry". Now enable expiry and pick an expiry date and go back to the edit screen.
Now toggle "Auto add to list after expiry" to ON as shown below. If you have never added this item to any shopping lists before, you will see a popup view asking to pick target shopping lists for this item.
Now you can enter the quantity of item you want to add to shopping list after the item expires.