If they don't refund you the difference and the item is already on its way to you in the mail, just refuse the package when the delivery guy shows up (you have to refuse it in person since alot of times the delivery guy will just leave a package at a doorstep if no one is home, which counts as being delivered). It will be sent back at no cost to you and should be automatically refunded when e-retailer receives it.
Most respectable e-retailers follow this rule. The only downside is of course you have to wait till they recieve the item back till you get your money refunded back. It's alot of waiting around just to replace an order if you are tight on cash.