That doesn't sound right. If you can, just place the order by phone in the morning.
As far as I know, for all the e-commerce software I've ever heard about, the price should be the same throughout the "shopping trip" except that some carts only show sale prices after you've added the item to the cart. It sounds like this is working backward.
Best to tell them what was happening. They could have some sort of error that they'll be able to correct, once you tell them about the problems you had.