I'd take any money saving announcement that comes through eBay or PayPal with a grain of salt.

When eBay announced their money saving shipping program advertising up to a 37% savings if a Seller paid for USPS Shipping going through eBay, I carefully compared the rate quotes to prices available by visiting the Post Office or pay online directly to USPS through the USPS Website. The rates from eBay are the same that you get directly from the USPS Online Site. There isn't any discount that you don't already get dealing directly with the USPS. And there was never a 37% discount from the USPS Customer Counter Rate.