Kraft envelope was too thin for heavy loading and was subject to tearing. I would pay more for a stronger and thicker paper. Staples used to have a thicker and stronger envelope available. I would suggest bringing this thicker product back and offering it as an alternative for those who want a stronger envelope. I would pay more for a better quality product. The second problem is the clasp. It also is too light weight. If the user opens and closes the clasp several times the clasp will break leaving the user with a worthless envelope. A stronger clasp goes with a stronger envelope !
It is often difficult to find protective envelopes made of quality material that are large enough to handle a full years worth of tax papers material for those all important deductions as well as the calculating documents used to get the final figures. Those factors must then be coupled with the ability of the overall package to fit into a filing cabinet that has the width for legal documents. These envelopes just happen to be the type that have all the necessary attributes to become "just what I needed."
This size envelope is needed to mailing a large number of pages of the usual 8.5x11 paper. The 9x12 is not large enough. This particular style includes an adhesive fastner rather than the customary. It might need scotch tape on the flap to reenforce.