And don't forget that texture, colour, shape and brand names can all be envelope 'messages'. Remember, the offer, or a hint of it, should usually go on the envelope, together with an indication that people must reply quickly. Don't forget that envelopes have fronts and backs and insides. If there's a window envelope, there's a space behind that. That should be used also.
Some mailers use the entire inside of the envelope to put testimonials on.
I have also seen it used as an additional order form. The letter comments, amplifies, makes more human, 'sells' the facts in the other material. People expect to receive a letter. They like getting letters. If you can't afford a costly mailing, then leave out the brochure not the letter.