Learn to count it back. It’s easy. “Yes sir, your order totals $5.67. Out of $20 (the amount the customer gave you) that’s 5.67, (starting with the pennies) 5.68, 5.69. 5.70 (now the nickel) 5.75 (now the quarter) $6 (now the $1 dollar bills) $7, $8, $9, $10, (now the single ten) $20.” (If you pay him back with two fives instead of the $10 bill, then it goes) ” ...$10, $15, and $20.”