You should never have to buy a new Mac and purchase the upgrade separately. All new Macs come with the latest version of the OS (either installed, or with a disc in the box). Period. The salesperson had to be mistaken, or they're trying to double charge you for something you shouldn't need to buy.
Either that, or they're selling you a really old 15" MBP. Regardless, a disc should be in the box. Insist on that, or purchase from an Apple store or Apple online.
--Travis