A friend of ours is getting married soon -- she and her mother have a difference of opinion on how the invitations are to be mailed out, specifically the tissue paper that comes with each invitation. Please settle the dispute.
Tissue paper is included with invitations so the ink doesn't smudge. But most invites these days are made with ink that really isn't going to smudge -- the tissue tradition continues more out of habit than for any other reason. Traditionally, the tissue is left in, but if it's really annoying your friend (or her mom) and isn't needed to stop smudging, it's fine to remove it before sealing and sending the invites.
Wedding Invitations + Wedding Stationery