You sure about that? I thought the free boxes were for anyone who ships Priority Mail. An ebay seller (or anyone else for that matter) who ships by any other method still has to provide/buy their own boxes.
It was for every seller, for a while. It may have changed since I quit selling regularly.
The boxes are for anyone using Priority Mail or Express Mail. There is no eBay special boxes. If you are shipping by either of these, you are already paying a premium which pays for the box.
I ship most things media mail and pay for my own padded envelopes.