Of course, if a seller doesn't care about losing Paypal disputes, and wants to investigate cheaper USPS shipping options,
here's the page for First Class Flat sizes:
Link Of course, if any of the following applies to your packages, it can NOT ship via First Class Flat:
Link a. Has an aspect ratio (length divided by height) of less than 1.3 or more than 2.5.
b. Is polybagged, polywrapped, or enclosed in any plastic material.
c. Has clasps, strings, buttons, or similar closure devices.
d. Contains items such as pens, pencils, or loose keys or coins that cause the thickness of the mailpiece to be uneven (see 601.11.18, Odd-Shaped Items in Paper Envelopes).
e. Is too rigid (does not bend easily when subjected to a transport belt tension of 40 pounds around an 11-inch diameter turn).
f. For pieces more than 4-1/4 inches high or 6 inches long, the thickness is less than 0.009 inch.
g. Has a delivery address parallel to the shorter dimension of the mailpiece.
h. Self-mailers with a folded edge perpendicular to the address if the piece is not folded and secured according to 3.13.1, Folded Self-Mailer.
i. Booklet-type pieces with the bound edge (spine) along the shorter dimension of the piece or at the top, unless prepared according to 3.13.