Tonearm - 10:08 pm on Dec 6, 2012 (gmt 0)

I've shipped thousands of small garments in USPS Flat-Rate Envelopes, but a customer contacted us today to tell us that their post office is charging them extra postage due to the fact that we "mis-used the envelope". Is anyone aware of a USPS rule that forbids putting anything other than documents in a USPS envelope?