Tyvek 9″ x 12″ WHITE Envelopes Box Of 100 Mailers TYF0912WS Open Side. #CE11 Purchase is for one box per purchase (100 envelopes per box.) Review images to see example of a box that will be shipped. Labeled TYF0912WS 9×12 white flat tyvek envelopes Envelopes as can be seen in a photo have a side flap opening.