How to handle a shared expense somebody disputes

Ask three things, in order: was it agreed in advance, does everybody use it, and was there a choice about whether to buy it. Almost every disputed expense is a clear yes or a clear no on at least two of those, and the disagreement is usually that each person is answering a different one.

Agreed in advance is the strongest test

Something the household discussed and decided together is shared, and something one person bought and then announced is much harder to defend — even when it was genuinely useful, and even when everybody now uses it.

Which is really a rule about process rather than about money: the way to make an expense shared is to ask before, not to justify after.

Use is the second test, not the first

Everybody using something does not automatically make it a shared cost, or every household would owe money for whatever the most enthusiastic person bought. But nobody using it settles the question immediately in the other direction.

And sometimes the answer is to pay it

For small amounts, being right costs more than being out of pocket. A twelve-dollar disagreement that runs for a fortnight has already cost both people more than twelve dollars in irritation, and the person who lets it go is not being a pushover — they are pricing the argument correctly.

Where it keeps happening, the problem is not the individual items. It is that the household has never agreed what counts as shared, and that is the conversation to have instead.