I am not sure if you can rescue to situation with the buyer.
You could try messaging them and be honest and say you didnt realise that you were still responsible for the items arriving undamaged.....even when in the hands of Royal mail and you have no control of how they handle them. If they have not already done so suggest they open a not as described case so you can refund them and pay for the return tracked postage through the case.
IF you can get back in the buyers good books they may agree to you sending them a Feedback Revision Request to change the negative. Saying that.....you must not and cannot make this a condition of you refunding them.
Hope the above helps.
