http://www.pixiv.net/member_illust.php?mode=medium&illust_id=15343784
http://www.taptaptaptaptap.net/otona-to-kodomo-no-merry-christmas/

I didn't understood the ending

Base +9, Kishi -10, Wilfriback -6, knightdtd +10

So they were both Santas, i think, and both of them know the truth. It is not about Santa is real or not, it is about the friendship between those two.

Short but nice.

Base +8, Kishi +10, Fake Hero -6, Wilfriback -6, SightlierGravy -6, knightdtd -10

"Santa appears before those who believe in him".
Despite how she mentions that, Risa doesn't actually believe in him, resulting in her not getting a visit from Santa.
On the other hand, although Miho doubts if Santa exist, she still believes that he does.

tl;dr, it's a story about childhood innocence and how people lose it whether they want to or not.

Base +15, aiwotorimodose +28, Kishi -10, Wilfriback +6, SightlierGravy +6, Fake Hero +7

So Risa's note ended up at Miho's house, and vice versa? Kinda confusing. No idea what that thing by Miho's letter is either.

Base +15, SightlierGravy -6

Man I never belived Santa exists. I always thought Christmas is about Jesus Christ. Santa was something invented by american capitalists in 70s, and its not by coincidance that its an anagram of satan.

Base +8