it is not a parcel...its a large envelope
USPS - First-Class Mail Large Envelopes
... Large envelopes exceed any one of the maximum dimensions of a letter. The maximum
size of a large envelope is 12 inches high by 15 inches long by 3/4 inches ...
www.usps.com/send/waystosendmail/senditwithintheus/firstclassflats.htm - 36k - 2009-06-08 -

so as long as its not more than 3/4in thick..it should go as a large envelope....88 cents if 1oz or under