During sexual intercourse right around the time of peak stimulation for the female partner. The male proceeds to insert a pre-written note inside of her vagina. Later, the female can pull it out and read what her partner has written.
Aaron: Did you finally tell that thot that you loved her?
Bill: Yeah man, I wrote it on a note and hit her with the Baltimore Mailbox!
At the movie ticket counter, Bobby reached into his pocket, pulled out a wad of money and flashed it for Shirley. Little did she know that was was a Baltimore Roll of one dollar bills.