Research shows that including an envelope with a reply paid address will dramatically increase your success rate when selling products by direct mail. It will also increase your return rate when sent out with accounts. All that is costs you is the postage and the cost of an envelope, yet very few small businesses use this simple promotional and customer service-oriented tool.
As mentioned in other tips, a key principle of customer service is making it easy for your customers to deal with you. Saving them time and money are two of the best ways to have your customers coming back for more. In this case, they don't have to find an envelope, then try to find the address that they have to send it to, then find a stamp and finally put the lot together. All they have to do is slip the information into the envelope provided and drop it into the post box next time they are passing by. Beautiful.