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Okay, but we all do agree that Stan and Ford send letters with the address where they're going to spend the Christmas to Dipper and Mabel so they can send them gifts and vise versa. Right?
I think that Mabel would send them sweaters that she made. Stan uses them when it's really cold (he pretends that he doesn't like them) and Ford it's always using them (after all, he also loves sweaters). Mabel always makes turtle neck sweaters for Ford because he showed her his tatto once (and she plans to do the same tatto because she loves it) and said that he wanted to hide it.
Dipper would send Ford a copy of a book that he read about paranormal stuff or some accessory of some movie/series/game that they both like and he would give Stan a key chain related to money or cassinos or a pair of pants.
Stan would steal a wallet from someone in the street and send the money he finds there (or in his brother's bedroom) to the twins or he would ask Ford to buy something for them for him. Or he even would walk into a shop and buy the first thing that he sees with glitter or animals and the first thing that screams "NERD" (or that puts Ford's eyes glowing).
Ford sends Dipper books that he liked to read when he was his age and he gives Mabel romance books that have colorful covers or something related to Ducktective.
Is this headcannon or cannon?
EDIT: I didn't know that the Pines are Jewish. So, instead of Christmas, they would buy this gifts for Hanukkah. They buy 8 gifts for eachother (out of the things I said) and they open them during the 8 days of Hanukkah :D