Custom stamps are the thing to have right now when putting that final touch before sending out your invitations but I just used these beautiful ones bought straight from the post office that I absolutely love. The 41 cents stamp collection is called "Beautiful Blooms" and I just used it for one of my brides who had us stamp her response cards as well as stuff the invitations.
The stamps feature different blooms in pinks, greens, oranges, and reds. It infuses just the right splash of color to brighten the envelope but the design is simple and subtle enough that it doesn't dominate it.
Don't underestimate how stamps can alter the feel of your invitation! The mailing envelope is the first thing your guests will see. It can further invoke the look and theme that you are trying to convey, or the wrong stamp can take away from all the work that you put into the beautiful package that will announce your big day.
Though I used these stamps for a fall client, I think that they will be perfect for upcoming Spring and Summer 2008 weddings too!