Did you get a copy of the receipt/signed paperwork?
See what it's like on Saturday, have a good look over. If you are not 100% happy then do not pay the balance, you are in a strong bargaining position and it will be much easier to sort out now rather than later.
One thing i would say as a bit of a 'btw' - if you are purchasing anything at all over £100, ALWAYS pay be credit card. Especially in these times of companies going under left, right and centre.