I ship daily and have for 8years now! The rules changing yearly are so annoying that it is making me close my online store this year!
HEre is a link to check out the rules for shipping.
http://pe.usps.com/cpim/ftp/manuals/dmm100/dmm100.pdf
Go to page #19 and print off pages 19, 4 & 5! use them as a reference when you come across a clerk who says it can not be more 3/4" thick. That is NOT listed anywhere on the USPS website! First class has to be 3/4" before you can put Delivery confirmation on it, but the flat rate envelopes are supposed to be if it can fit, it ships. And it is recommended that you use one piece of tape across the top!