Correct me if I'm wrong, but I think you can mail just about anything via the usps blue box (the kind you find on the street corner), as long as it is under 13 oz, stamped correctly, and can fit in the little door.
Giverslog dedicates a whole section in their blog on how to mail items such as bouncy balls, flip flops, silly putty, etc. in creative packages (brown boxes and envelopes used sparingly).
Don't you want to get something fun in the mail now? Thank you Giverslog for the wonderful ideas!