USPS Parcel Post has two sets of rates, depending on whether your package is "machinable" or not.  (Machinable packages can be handled by their automated systems; nonmachinable machines must be manually handled).

A machinable parcel is any piece that is:

a. A parcel more than 34 inches long, 17 inches wide, or 17 inches high.

b. A parcel less than 6 inches long, 1/4 inch thick, or 3 inches high.

c. A parcel that weighs less than 6 ounces or more than 35 pounds.

In order for AuctionInc to be able to procure the machinable Parcel Post rate from USPS, we must be able to send them package dimensions, which, in turn, we can only do if you are providing us with your item dimensions.  Thus, if you are offering Parcel Post as an option to your customers, it is best if you include your item dimensions in your item data.