This is where I buy my mini envelopes. I usually buy the white envelopes, but they have other colors, too.
I include an envelope when I ship out my soap orders. I can fit a regular size business card, a moo card and a soap sample in the envelope. I like doing it this way, because it's a more fun look than just tossing everything in the box. Sometimes I tie them with raffia or funky yarn:
Monsterpromos also sells customized totes, aprons, banners and bumper stickers and more. They ship quickly and are completely professional. Be sure to check them out.