Keep the Change

Karina and I went to the Dollar store so that Karina could buy two of her favorite things in the world.  A coloring book and some glue.  We got in line to pay and I gave Karina a ten dollar bill to pay for her items.  The grand total was $2.19.  Karina handed the cashier the ten dollar bill and said, "keep the change". The cashier seemed very surprised but also very grateful.  What could I say?