that makes more sence now. though they may say they gave you the best deal they have, by signing up to one you get a package discount on other. though im sure you have a cool off period for the contract regardless of if its in the shop. bad news is it may be 14 days.
you havent said anything thats "wrong". you've a mobile tariff and a mobile phone effectivly on HP. unless you're saying the 15GB tariff is too expensive, whats the issue?