Goodbye, mother

Walking through a supermarket, a young man noticed an old lady following him around. He ignored her for a while, but when he got to the checkout line, she got in front of him.
“Pardon me,” she said. “I'm sorry if I've been staring, but you look just like me son who died recently.
“I'm sorry for your loss,” the young man replied. “Is there anything I can do for you?”
“Well, as I'm leaving, could you just say ‘Goodbye, mother!?' It would make me feel so much better.” She gave him a sweet smile.
“Of course I can,” the young man promised.
As she gathered her bags and left, he called out “Goodbye, mother!” just as she had requested, feeling good about her smile.
Stepping up to the counter, he saw that his total was about $100 higher than it should be. “That amount is wrong,” he said. “I only have a few items!”
“Oh, your mother said that you would pay for her,” explained the clerk.