Good question, the shipping calculations and options can often seem confusing. In this case I believe the answer you received from eBay is correct.
For Auctiva shipping labels, we also calculate the "Girth" (or the distance around the package) with a formula so you only have to enter the more intuitive values of length, width , and height. Our formula to Girth is essentially:
(2 x (package width + package height)) We then add length to that value to ensure the package meets the 108 inch combined package length and girth limit (ref. http://pe.usps.com/businessmai...ristics/parcels.htm).
here is a reference that may also explain in more detail: http://www.ehow.com/how_823719...late-girth-usps.html