The post office is having issues with the bulging envelopes that are taped end to end, corner to corner so that no part of the envelope has been left without tape. However they will accept the flap being taped and the label being taped. even if you have to call a manager- the reason you should give is: you are making precautionary measures to keep the flap secure & mailing label intact during the shipping process.
I use flat rate all the time.. you just have to be able to seal the envelope on its own first prior to the taping. I can send a pair of jeans in them.. Yes A PAIR OF JEANS!!!!
Their motto for flat rates is: " if it fits it ships"