You can eliminate paper waste, if you are standard in your mailings. Standard sizes go for envelopes too. Odd shaped envelopes may be attention-getters, but they have to be custom made, at very high cost. The advertiser's answer to a letter should take into account the express wishes of the inquirer. Use the YOU approach in your advertising copy.
Most people read direct mail newsletters.
They keep people up to date and interested. You can speak in a sales pitch, but be subtle. Use colored paper. Printing on colored stock with two different color inks can create the effect of a three-color job, without the expence. Plan to have several different pieces grouped together and printed at the same time, in the same colors, and on the same paper stock.