If you use Business Reply Mail - you can use your Endicia software to design your BRM mail pieces.

While most CASS-Certified BRM Design applications can cost over $300, Endicia includes it with Premium and Professional accounts. Whether you are designing a BRM postcard, label or envelope the Endicia software will provide artistic freedom – while adhering to the strict guidelines of the US Postal Service.
If you don’t already use Business Reply Mail, now might be a perfect time to start. With BRM you’ll open up a whole new channel of customer communication. Along with your Endicia account, you’ll need a Business Reply Mail permit from USPS. For pricing on permits – go here: http://www.usps.com/replymail/rates.htm