When drafting the copy, look, feel and offer for your mail package, consider the 3 S's:
Simplify. Do not use this opportunity to brag about your company or include essays about the firm's history. Inform your customers about the benefits of one item or service. Don't confuse the customer with choices.
Specify. Tell him about the offer upfront (and preferably on the envelope). Then tell him again. Spell out details — clearly.
Showcase. Put yourself in the customer's place. Is this an offer you'd respond to? Make sure it's foolproof.