Think all paper is alike? Not so, and it makes a difference when your mail goes to the Post Office. Post cards must be at least .09 pt. but paper from different mills comes in different. It may say that it is .09 but when we put a micrometer on it it does not met postal specifications. The solution to this is that we always use .10 pt. or higher for our post cards. Another requirement is if mailing an 8.5×11 fold to 5.5×8.5 the paper must be 100# for gloss and 70# for un-coated paper. If mailing Standard A (bulk) all of the paper in the mailing must be of the same weight throughout the mailing. Having different weights of paper may have the mailing turned down by the USPS.
The best thing to do is check with experts, such as Burns Mailing & Printing, Inc. We take care of all the requirements for postal specifications, printing and mailing.