Tag where to buy something right as you add it. Type the item, then "from" or "at" and the store. "Milk from Costco" adds milk tagged for Costco. "Eggs at Trader Joe's" works the same way. A small "Adds to Costco" pill shows above the bar so you can see it land.
If the store is new, Stores offers to add it. Tap to confirm, or press return to keep the words as plain text. The tagged item then shows a store badge on its row.
This works on a task list too. There the badge is a tag and the wording fits to-dos, but you type it the same way.