There's an Always Sunny in Philadelphia episode that covers this question, if you've seen it maybe you will remember the big "H" on the box is for "Hornets" so we all know it's a box full of hornets...

I mean, you could try mailing someone a box of hornets with a big "H" on it and tell them it's a present and the H stands for "Happy Housewarming," but you were talking about legitimate reasons to mail bees, right?