Cannot use the term "rip off" cos they did not cheat nor did they entice or coerce you to buy. You wanted the stuff. They quoted you the price. You paid up.
What you can do is go down to the shop, ask to see the same sales person and ask him to quote you a pice for the same stuff. If it is lower, then ask him to reimburse you the difference since you claimed that you have overpaid for it.
If after confronting them nicely, they still do not want to reimburse you the difference then your next logical option is to claim the full sum from the "Small Claims Tribunal". This is what they call the cooling off period.
John 3:16 is always a bit more ex. Sometimes 10 - 15% more than the others. Everybody knows that. But we still want to buy from them because they are so friendly. They want to make you feel special so that you will part with your money readily.
In making your claims, remember to keep your cool and ask nicely. Customers who showed their colours when they feel they are in the right will rarely get their money back.
Hope that helps. Let us know how it goes.