[header] How to give back correct change [title] Make sure the change you give and the price of the product together add up to the cash the customer gave you. [step] You want the customer to leave with the same value they gave you, only now part of that value will be the product, and the rest change. It's that simple. 
For example : [substeps] If they gave you $20 to buy a $5 book, they will leave with $5 worth of book plus $15 worth of change, for a total value of $20. [title] Add up the amount the customer gives you.