If you’re hoping to get customers to tip 18 or even 21% on their purchases, try showing them how much they’ll be charged if they pay the extra amount.

For instance, on a purchase of $20, display that 15% = $3, 18% = $3.60, and 21% = $4.20.

When I saw how similar the amount tipped at the rate of 15% would be to that of tipping at the rate of 18%, the annoyance of tipping more than 15% quickly faded.

Worth a shot.