A return receipt is sort of an ancillary ancillary service for
US Mail - it is used as the "return receipt" in
return receipt for merchandise, is also used in addition to
certified mail,
COD,
Insured mail,
express mail,
recorded delivery, or
registered mail, and is an item that provides physical proof that your item was delivered.
The item itself is a mint green postcard (or salmon for international mail), and is affixed to your article in conjunction with the above services. The return receipt is affixed with your return address on one side, and various bits of data filled out on the exposed side; on large items, it is placed on the same side as the address information, on smaller items it is placed on another surface and the words "return receipt requested" are generally placed on the front of the article as a reminder to the carrier that there is something requiring a signature here. Sometimes it doesn't come back - because sometimes the carrier forgets, and that's generally because those three magic words are not on the front of the article.
Cost for this as a conjunctive service (that is, as used for anything but return receipt for merchandise) is $1.75.