OK, direct from the horses mouth, i.e. the USPS Mailing manual -- Read it yourself, or if you're having trouble with your local postal clerk, print it and take it to them...
http://pe.usps.com/cpim/ftp/manuals/...PDF_5#xml=http://pe.usps.com/search/jsp/search/xmlread.jsp?k2dockey=http%3A%2F%2Fpe.usps.com%2Fcp im%2Fftp%2Fmanuals%2Fdmm300%2F423.pdf%40PE_DMM300_ PDF_5&serverSpec=56.0.145.56:9920&QueryParser=Simple&querytext=flat-rate%3Cor%3Eflat%3Cand%3Erate%3Cand%3E+envelope+pa cking
Please note: you'll have to cut and paste the url above into your browser, as for some reason the QB forum software won't allow me to post a shortened version of the url.
Here's the relevant section -- 1.7 --
1.7 Flat Rate Envelopes and Boxes
Any amount of material may be mailed in a USPS-produced Priority Mail Flat Rate
Box or Flat Rate Envelope. When sealing a Flat Rate Box or Flat Rate Envelope, the
container flaps must be able to close within the normal folds. Tape may be applied
to the flaps and seams to reinforce the container; provided the design of the
container is not enlarged by opening the sides and the container is not
reconstructed in any way.