Here's my question. If a merchant changes their terms and conditions after the point of sale, can you request a chargeback?

Here's the scenario. I purchased goods online and agreed to the terms and conditions. I finished my order and listed a Po Box for my delivery address.

The merchant has then contacted me and advised they will not deliver to a Po Box as per their terms and conditions.

As their terms and conditions don't state anything relating to a Po Box, I advised them it was not in their terms and conditions and requested the order be cancelled and refunded as the Po Box is our only suitable delivery address.

The merchant then advised they would only offer a partial refund as per their terms and conditions and sent me an adjusted terms and conditions with an added Po Box clause (very unethical).

I then attempted a chargeback but it was disputed by the merchant who claimed we agreed to their terms and conditions, which the bank accepted. The chargeback was then declined.

Any thoughts?