I would just politely tell them that you got a quote from FedEx for $51 per seat and ask if they would be willing to ship them that way. If not, ask if it's okay to arrange your own shipping if they will get the seats ready to go.
I guess the absolute worst that could happen is they could file a non-paying bidder complaint against you and they could leave you a negative mark on your feedback. I doubt Ebay will do anything about it since it does say "contact them" or whatever. Just looking at it from Ebay's eyes, I'm sure they will say you should have asked about shipping costs before you placed your bid.
Don't take that personal, it's not a dig at you at all. That's just what I would lay money on being Ebay's point of view. I definitely think it's BS and I see it all the time. Undoubtedly they knew freight would be high on these and they should have pointed it out in the description.