I was going to label this post 'How to: Sting Tie Envelope Closure' but really, you can apply this fasterner/closure to much more than just envelopes.
First you will need an envelope template, envelope maker or anything you have to create the envelope. Most time efficient and cost efficient would be taking apart an existing envelope and tracing.
Other supplies: circle punch, eyelets, eyelet setter and wax string. Well really, you can use any time of string, floss, yarn, etc. but I prefer the wax string which you can find in the jewelry/beading department of most craft stores.
After you have traced and scored the envelope, punch two circles from a co-ordinating piece/scrap of cardstock. Do not use light weight cardstock. Because the string has to wrap under the punched circles, you need a heavy weight cardstock for the closure to last more than for a few opens/closes :) Fold the envelope as it will appear finished and mark the center on both back flaps to indicate where to punch the eyelet size hole.
Repeat the same on the other flap but omit the string. For the opposite flap, you need one punched circle and one eyelet. Presto and voila! you have a string tie closure on an envelope! The same principle can be applied to fabric using real buttons but you would instead sew the string to the area under one button. A shank style button works well but regular buttons will do a great job, especially if the 'holes' are towards the center of the button and not near the edge of the button.
If creating the envelopes yourself doesn't work for you, you can simply visit my Etsy boutique where you will find a nice assortment of handmade envelopes with either string tie closures and adhesive strip closures.
Thanks for stoppin' by!