It is only because you did not use the systems in place, you should have opened THREE not as described cases, seller then has to prove THREE reply paid labels and refund in full on return, or just refund in full.
There is no way to make seller honour any promises made using messages.
It does pay to use the ebay 30 day money back guarantee, it is on every listing on the right, and it is intended to be read, and used by buyers.