Your friend Sam didn't get the joke. Can you explain it?

Cartoon: C. L. Bartholomew, Uncle Sam's Special Delivery for Santa Claus Letters (1912), public domain, via Wikimedia Commons
The situation: you've just told the joke to your friend Sam. Everybody is laughing… except Sam: “I don't get it. Why does Tommy call them losers? They were so nice to him!” Explain the joke to Sam. Answer each question aloud with the key words first, then click Show a model.
“Wait… who sent the presents?”
Explain with:
Well, the presents didn't come from Santa Claus. The post office workers read Tommy's letter and they felt sorry for him, so they collected about eighty dollars and bought the presents themselves.
“But Tommy thanks Santa Claus!”
Explain with:
Exactly! Tommy doesn't know that. He still believes in Santa Claus, so he thinks the presents come from Santa.
“So why didn't he get the gloves?”
Explain with:
The workers only had about eighty dollars. They bought a coat, a hat and a video game, and there was no money left for the gloves. The story doesn't say it: you have to guess!
“And why does he say they stole them?”
Explain with:
Tommy knows that his letter went through the post office. So he thinks Santa sent four presents and the workers stole the gloves on the way. He blames the people who helped him!
“Oh! But why is it funny?”
Explain with:
That's the irony: he insults the very people who were kind to him! The whole story is sad and touching, and then the sweet little boy calls them “incompetent losers”. We don't expect it at all: that's the punchline.
One of you tells the joke, the other plays Sam and asks the five questions, in any order. Then swap roles. Don't read the models: explain with your own words.