I just visited the USPS website, and found this information under "A Customer's Guide to Mailing".
This is cut and pasted from the site in it's entirety.
Note: Flat Rate prices are available when material is sent in a USPS-produced Flat Rate Envelope or Box. 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.
Weight restrictions for domestic Express Mail and Priority Mail Flat Rate Envelopes and Boxes are 70 pounds.
Weight restrictions for Priority Mail International are 4 pounds for the Flat Rate Envelopes, Small Flat Rate Box, and DVD Box and Large Video Box; 20 pounds for the Medium Flat Rate Boxes and the Large Flat Rate Box.
Weight restrictions for Express Mail Flat Rate International vary by country and are noted in the Individual Country Listings. However, the maximum weight limit for any Express Mail International shipment is 70 pounds.
Rate restriction for both boxes and envelope is 70 lbs and mentions nothing about what the contents must be. No restrictions there. I will print this out and take it with me when I go!