You're right it doesn't cost that much to ship, but like Joe said, the eBay fees, Pay Pal fees, and actual shipping price, DOES cost that much.
So lets say he buys a ball for $50...you win it on eBay for $50...they take away the sellers fee...then the final value fee, then you pay with pay pal, and they take a part of that too. He only charges the actual price to ship, now he's in the hole. Doesn't make too much sense does it?
If you wanted to buy a ball from him, why not just do it over BR?