What Does It Mean?

 

I spotted this sticker pasted to a light pole in a parking lot and the eagle caught my eye.  That is the symbol or trademark for the U. S. Postal Service.  Normally, it would say U.S. Mail.  Why it says "Moist" is a mystery to me.

I tried doing several searches online to find a reason for the sticker but got no useful results.  One explanation said something about stickers being made for water bottles and laptops and another just said it was a parody. But why?  Obviously it cost someone to create the thing so my question remains, why and what purpose does it serve? 

Maybe it's a joke that I just don't get.