If there has there is no mention of it on the USPS website.
Here is what it says:
Priority Mail
A simpler way to ship.
Take the guesswork out of shipping with Priority Mail® Flat Rates. Whatever fits in the box
or envelope ships for one low rate—anywhere in the United States. There's virtually no weighing or calculating. And your packages reach their destination in 2-3 days.
Restrictions
Contents must reasonably fit within the Priority Mail packaging, and weigh less than 70 pounds
Link:
http://www.usps.com/shipping/prioritymail.htm