I purchased stales® 9" x 12" Brown Kraft Clasp Envelopes 12/Pack at my local store. I was able to send all of my necessary documents in standard sized paper within the envelope. The seal stayed stuck and the metal clip provided extra seal age as well. I was also advised by the postman to seal the envelop with scotch tape also. Then he marked over the seal and tape in order to provide additional protection. If the envelope was compromised during shipment it would be fairly obvious.
I purchased the Staples brand 9X12 clasp envelopes for a mass mailing to potential clients. The price was almost a dollar less than another brand. The quality of the envelopes is much better than I had anticipated for a store brand.
Seems like I have had a bunch of mailings of 4 or more pages and these are just right for mailing those oversize letters and documents. Trying to stuff 5 or six pages in a #10 envelope is a pain and looks overstuffed.