When I'm sending them furniture I keep it brief and say something like "Please use this" or "Please wear this" if I'm sending them clothes. The exception is the first letter I send to a new villager, if the first thing I'm doing is sending them a new shirt I'll make it look like a warning letter from the Fashion Police.
I only really send letters while a villager is still new. Usually it'll be to ask them stupid questions or make dumb references like "WHY PENELOPE'S EARS BOW? FIND OUT NEXT TIME ON DRAGO BALL Z"
I try to keep my initial letters entertaining as a nice easter egg for a villager's new owner to find. Sometimes I mess up.