IANAL so this is just my own perspective. You may want to ask over at Law.SE for a more definitive answer.
The fact that this merchant went ahead and shipped goods without checking their new terms by you first is their problem, not yours. If they didn't want to exchange goods under the original terms then they could have withheld them until you had reached another agreement, or cancelled the transaction immediately. Nobody forced them to send out a package. If you don't accept their new terms then you won't own whatever shows up on your doorstep and nor should you be charged a single penny for it. You may have a duty to ensure the package is not subject to any increased risk of damage or theft, but other than that, the cost involved in getting the item back to the merchant will be on them (serves them right for jumping the gun). If the cost to recover the item is $43 or more then it may be in their best interest to let you keep it for the original price.
If they persist and collect the $270 from your credit card, you are within rights to dispute that charge. It would be an unauthorized charge after all. You have proof in your email inbox that the agreement was for a smaller amount. Whether the CC company would try to reduce the merchant's charge to the authorized amount, or have them remove it altogether, I'm not sure. Provided you explain your situation clearly, they should be able to advise how best to proceed.