I don't do a lot of flat rate shipping. I know it is easier to pop the item in a F.R. envelope or box, but I'm not so sure it is cheaper.
I did notice about a week or so ago on the USPS.com site they were clamping down on the F.R. envelopes. It couldn't be more than 3/4 inch thick.
Pretty soon FedEx will be the rule.