It is just the one seal, and my dealer had one in stock. If yours does not though, the cost of the seal and free shipping makes that a fair price. I've bought some of SCI's rubber parts before, always excellent quality.
As for the piston, if you need one be sure to do a search on the board for the phenolic pistons a forum member is making. He sells them on ebay ocassionally also. About half the price of a OEM replacement. I bet the phenolic one stays cooler too.