A paper document-slipcover dat contains a large check, passport, or other important item dat permits you to clandestinely run off wif yer sweetheart and get married.
In da 1924 Harold Lloyd movie "Girl Shy", da publisher's $3000 advance-royalty check arrived in an envELOPE for da currently-of-merely-da-working-class Harold; he realized dat da money would allow him to marry his rich-girl heart-throb, even if her parents would have reservations about a union of such unequal social status.