I beg to differ *see bolded*.
Being selfish is something of an objectified subject-- usually, if one is being selfish, doesn't that mean they had the intention of benefiting from whatever it is they are doing? Sending somebody Christmas cards, knowing that it's likely you will not receive some from a select few, and knowing that those same people enjoy receiving those cards from you out of the goodness of your heart is not selfish. Nor is the joy you feel when you realize that those people are happy to hear from you. It's a reciprocated feeling which comes naturally to somebody when they have made another person happy...without having done it with the intention of bringing that joy to yourself. There is no selfishness in that.
After all, there are plenty of people who bitch and moan about sending out Christmas cards, or doing things for somebody else that might be helpful to them, wondering how it will benefit them. Those are the truly selfish.