The United Stated Postal Service actually has great resources for setting up you campaign to be cost effective in their “Grow Your Business” section of their site.
http://usps.com/directmail/welcome.htm?from=0001home&page=directmail
You can also make an appointment to meet with a represtative. They will also supply you with sizing proportions, bulk rates, barcoding, permit numbers, specs and postage templates for you campaign. You will need to eventually cantact them your your postage rates and reply card information. You will also need to get the acceptable dimentions from them… if you design your direct mail in the wrong proportions you may eat it in postage costs. A great example of this is that square mailers tend to jam the machine and need special handling… which can cost a few cents more per piece. Multiply that by a 40,000 piece campaign (double with a postage paid reply card) your deep in costs… well you may get 5% of the reply cards back but that’s still a heafty amount you could save if you research from the start.
They are really helpful… they want your business.