Supermarket Mother

A young man was walking through a supermarket to pick a few things when he noticed an old lady following him around.
Thinking nothing of it, he ignored her and continued on.
Finally, he went to the checkout line, but she got in front of him.
"Pardon me," she said, "I'm sorry if my staring at you has made you feel uncomfortable. It's just that you look just like my son who just died recently."

"I'm very sorry," replied the young man, "Is there anything I can do for you?"

"Yes," she said. "As I'm leaving, can you say 'Goodbye mother'? It would make me feel much better."

"Sure," answered the young man.

When the old woman was leaving, he called out, "Goodbye mother!"

As he stepped up to the checkout counter, he saw that his total was $127.50. "How can that be?" he asked, "I only purchased a few things!"

"Your mother said that you would pay for her's," said the clerk.