You do have to keep checking what the rates are. There are new "Regional Rate" boxes that weren't available a few years ago, plus while we always hear in the news that USPS is raising the cost for a first-class stamp, when they raise parcel rates there isn't nearly as much talk about it.
At least there hasn't been any trouble with sending stUFFed flat-rate envelopes in over a year. They are sticking by the "if it fits, it ships" slogan (provided that the envelope does not exceed the weight limit).