I definitely suspect big-hearted postal workers at somewhere along the line - maybe multiple places!

(This is one of the reasons that I tend to put holiday sticker art on my card envelopes. They can pretend that a red or green card-sized envelope is just a birthday card, but when it is tricked out with candy canes, holly, polar bears wearing mufflers, silver stars, etc. (and dreidels and such on the Hanukkah ones) , it's hard to pretend that you don't know it's a holiday card!)