First, go look at the documentation on their website and that you received...I'll give 10-1 odds, they have a clause that they can adjust the rates based on updates to pricing.
If they can produce that clause, you are contractually responsible for the extra. Depending on the exact language, they might not even need to issue a refund.
If they can't produce that, you can tell them no but if you do that expect at best a very disagreeable visit or they might refund the money and tell you to bugger off...they know you aren't going to sue them.
Poor business practice but not much you can do about it.