I got stung on this to the other day. I mailed approximately 12 6x9 envelopes over a 3 day period all with 1st Class postage. One came back asking $1.13 for 1st Class Parcel Post. According to USPS.Com, 1st Class Parcel Post applies if the envelope exceeds 12x15 or is thicker than 3/4". There is also language in there that the thickness must be uniform.

My envelope was less than those requirements. My wife took it back to the PO for me and the clerk used the non-machinable template (1/4")to decide that indeed it was a Parcel. Bottom line is that the PO clerks have a high error rate and you shouldn't take their word for anything. You would think that they might have problems on obscure shipments (2 cats to China), but they screw up the simple stuff.