The Post Office has created an enormous document of mailing requirements that will answer almost every postal issue question you can imagine.  See  www.usps.gov/malingrequirements.  If the document appears overwhelming we are here for you to answer your questions and get you in the right direction for mail piece design. The last thing we ever want is to print a mail piece only to find out that some issue about the mail piece will cost you extra postage that could have been prevented. If you have designed your own mail piece and you want to get approval of your artwork for Postal Regulations, you can email your artwork to MDA@usps.gov. A mailing requirements specialist will review your artwork and let you know if any changes need to be made or if it is ok as is.