So I found the cutest envelopes and they're DIY (or buy for $$$ on etsy). All it takes is this pdf which you should print out on cardtock and a picture you love, like an 8x10 image, a page from your favourite book, etc. This template will make 5.5" x 4" envelopes.
Trace the cardstock template on the picture/page you chose and cut it out. Glue the edges and fold in place. For easy step-by-step instructions, visit DIY Weddings.