The above labels are free Suzee Que Vintage mailing labels. If you click the link, it will take you to more labels and instructions for downloading and using them. These would look great on invitation envelopes.
If the above are too ornate for your taste, there is the Lil' Prettie mailing label from Le Blagh (the blog may have moved, but all the previous posts are still there, including some lovely free downloads). It has a simple vintage flavor.
If a wrap label (a label that wraps around one side of the envelope) is more your style, there is the French Postcard wrap label from Le Blagh (subtitled Pretty Procrastination) as well. The blog post also includes a free bookplate download at the bottom.
The Antique Ledger wrap label, also from Le Blagh, is simpler than the French Postcard wrap label, and could showcase a love of math and vintage.
The Ming wrap labels and mini Ming mailing label are more contemporary looking, but could still pass for vintage.
Wrap labels will need to be printed out on a piece of paper and then pasted onto envelopes. Depending on how talented you are with the computer and printer, the mailing labels may be printed and pasted, or printed directly onto the envelopes. Most of the labels will need the addresses handwritten on unless, of course, you can figure out how to get the addresses typed inside. Be careful with the first set of labels-- they are set to prepopulate the rest of the labels on a sheet once you have typed in the first label on the sheet.
If you want to play around with just decorating your envelopes (or your own DIY invitations), check out the Graphics Fairy.
