Label sample items with prices. To keep things simple, use prices that already look like sales tax has been added. Use a variety of prices, ranging in difficulty from round numbers ($1.00) to odd prices ($3.72).
Pair off students and have them take turns being the cashier and the shopper. The shopper is given five- and 10-dollar bills, while the cashier is given a variety of smaller bills and coins.
Have the shopper choose an item and pay for it with a single bill.
Instruct the cashier to take the bill and count back change, starting with the purchase price and ending with the amount of the bill. Start with the smallest amounts and step up to higher denominations every time he hits a round number. For example, if Jane buys a teddy bear for $1.67 with a $5 bill, Ricardo will count back three pennies ($1.70), three dimes ($2), and three dollar bills ($5).
Alternate until all the items have been purchased once by each player.