Why does the Post Office always lose money?

This hilarious story from the Consumerist provides a clue:

The Styrofoam cube enclosed in this envelope is being included by the sender to meet a United States Postal Service regulation. This regulation requires a first class letter or flat using the Delivery or Signature Confirmation service to become a parcel and that it "is in a box or, if not in a box, is more than 3/4 of an inch thick at its thickest point." The cube has no other purpose and may be disposed of upon opening this correspondence.

Alec Baldwin: They're just sitting out there, waiting to give you their money! Are you gonna take it? Are you man enough to take it?

United States Post Office Worker: No! Not unless their money is in an envelope at least 3/4 of an inch thick!

Alec Baldwin: *picks up set of steak knives, proceeds to stab Post Office worker in rage*