Returned 2 shirts and only got credited for one

[sufferingicecakes]

I bought DH two designer shirts - they were clearly fakes, but they did not fit either.

The seller offered full refund if not happy, so I contacted and she sent postage label. In my message I thought it was clear that I was returning both. I returned the shirts (admittedly all a bit hurriedly as we were off on holiday) but then I only got credited for one of them.

To make matters worse, all the correspondence and admin details have disappeared from my account on this one.

What can I do? How can I prove that I returned both. If I have to chalk this up to experience, how do I ensure this does not happen again?