If you want to print a uniform screen or solid accross the varying layers of an envelope, stick a layer (or 2 ) of Duct Tape on the impression cylinder and create a Back-Packer.
A layer of compressable Duct Tape behind the image area conforms to the thicknesses of the back-side of the envelope producing uniform back cylinder pressure against the blanket.
To position Duct Tape accurrately, trip impression cylinder and create an image on back cylinder, then stick Duct Tape over the image area.
Before printing, back-off impression cylinder and run an envelope to evaluate print image quality.
If printed image is incomplete, adjust impression pressure to higher setting until you get that perfect image.