Hi doublest,
Thanks for the clarification
Ebay also does not offer the option to enter the “girth” when entering your package dimensions during the listing process, so I not believe it is possible to ensure that factor is taken into account when selecting the calculated shipping option.
I would not expect the “girth” to make much of a difference in most cases since it is so closely related to the “width” and the “height”, but I'm sure USPS must offer that option on their site for a reason so I certainly understand your concern.
At this point, I recommend contacting eBay's Customer Support team to see if they have any ideas on how best to use the calculated shipping option for nonrectangular packages.
-Mike