After assembling an invitation my fiance decided that he wanted to incorporate a wax seal into the invitations, so that's what we're going to do.
We plan to do a dark purple seal on the inner white envelope for some nice contrast. Before finalizing our decision though, we plan to do one and then send it to ourselves to make sure that it doesn't get ruined in the mail. When we get to that step, I will let you know how it goes!
Note: Do NOT put a seal on the outermost envelope. It will get caught in the machines at the post office, causing your envelope to wrinkle or rip.