You can seal the envelope by creating your very own stickers.
Using the same template that I provided in step 2, design your seal.
Print them out on sticker paper. I bought mine at Staples. Using your 1.5" hole puncher, punch out the stickers, and attach them to seal envelope. You're all set and ready to send them out!
Be proud that you made your invitations, 100% on your own. If you are truly hardcore, you will give these a try.