Party-tax

The "tax" you pay to the host of a party, in form of refund on brought booze.
In countries with a refund system on bottles and cans, it is an unwritten social rule to leave your refund at the party. Party-tax-evasion is considered a very douchy thing to do and can eventually result in social suicide

If you help with cleaning up tomorrow, I'll buy you breakfast with the party-tax.