When I stopped sending out large sample packets , I was left with a ton of 9 x12 envelopes. I found they have become a great way for me to manage projects.
When I get an assignment, I label an envelope with the assignment title and deadline. This then holds printed versions of the layout, specs and sketches.
I also use them for individual illustrations to hold cut pieces of felt so they don't get confused with other assignments. I tape a printout of the sketch on the outside to easily find which is which.