Mailing
Traditionally, wedding invitations are mailed in double envelopes. The inner envelope may be lined, is not gummed, and fits into the outer envelope. The outer envelope is gummed for sealing and addressing. More recently, the inner envelope is often left out in the interest of saving money, paper, and postage. In some cases, a pocketfold takes the place of an inner envelope.
In countries that issue them, the envelope may be franked with love stamps. The United States postal service issues a love stamp each year specifically denominated to cover the double weight of the invitation and reply (a rate slightly less than the cost of two regular stamps).
Read more about this topic: Wedding Invitation