Tip jar bills

ONES

Think of the smallest paper currency often dropped into jars for service workers—these are the bills you might use to say "keep the change.

Tip jar bills" refers to the small denominations people often drop into jars for service workers. The answer "ONES" fits because one-dollar bills are the most common paper currency used for tipping. The clue plays on the literal meaning of "bills" as currency, not invoices, making it a straightforward definition.