Start with a week of real purchases
Go through one actual shopping trip rather than imagining everything you might buy together over the next year. Ask which purchases both people expected the household to cover when they left for the shop. This question often brings out a difference that a category name such as “food” hides.
Who chose the extra cost?
Consider coffee. If the household usually buys a $9 bag but one person wants a special $15 bag, you could treat $9 as shared and the $6 upgrade as personal. You could also both choose the upgrade. Discuss which applies before picking up the bag, especially when the other person will do the shopping.
Write a rule that both people can use
For the next uncertain item, ask before buying it. If somebody has already made the purchase, let them explain what they expected and agree how to handle this particular receipt. A new rule is easier to use when it starts with the next purchase.
A useful note might say “Household covers ordinary cleaning supplies. Check first about supplies for overnight guests. Personal toiletries stay personal.” Try it for a few shopping trips and revisit only the entries that were hard to categorize. Each adult should be able to use the rule without repeatedly asking the person who wrote it.