PC Performance comes as 2 types. One is a one off payment of 9.99 or 19.99 depending on the products value which last for usually 3 years. The other is a direct debit varying from 1.99 to 9.99 per month and lasting for as long as you keep up the payments.
1) Did you sign for a direct debit? If yes then maybe you should have questioned or at least read the paperwork you were signing. If no then any PC Performance must have been the one off payment type.
2) Was the total amount more than the products cost i.e. was the PC Performance added as an extra cost? If it was then the till operator was in the wrong (but it does say check amount & enter pin!). If there was no extra charge (you said money was discounted) then you have got 3 years PC Performance for free, in which case I can't see a problem.