These sturdy little envelopes are just right for mailing letters that are just a bit too bulky for a regular letter sized envelope. We always make sure to have a box on hand in our home office. You should too!
I have been using these for several years. They are durable unless you insert something other than paper. My only complaint is the clasp. We use ours for mailing and I would prefer that they have only the adhesive. The clasp tends to keep the flap from sealing around it. I have to put a piece of tape all the way across it. It would be more convenient for our purposes if the flap could fold over and seal and I would not have to take time to mess with the clasp and tape it. Depending on your needs this might not be an issue.
You really can't beat the value when it comes to 6x9 clasp envelopes. There's an adhesive but our office uses glue sticks for all envelopes. The clasps are very sturdy and stay secure. There is absolutely no transparency so your documents are secure.