It's a very controversial topic on this board.

As a practical matter, you could treat it as a letter (and add the 20-cent non-machinable surcharge) and drop it in the mail box (don't hand it to the clerk), and it should be all right 99% of the time if it is not over 1/4" thick. Must be no more than 3.5 ounces for letter rate.

Some clerks will automatically call it a first class parcel.

Have fun reading this:
http://pe.usps.com/text/dmm300/101.htm#1002718

I made this to check the thickness:

Robt.
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Deborah Kerr placing a bet at a roulette table, as Robert Walker and Peter Lawford look on, in "Please Believe Me" (1950).