Can a seller do a partial refund while I have a case open for item not received?

[awesomegirlstuff]

Hi

My ordered goods are at the post office as insufficient postage was put on it. Having read advice on these boards, I contacted the seller who offered to refund the fee and charge so I could then pay it and retrieve my item. However, seller says she has been unable to do a partial refund as when I opened the request I asked for a refund and it automatically assumed the whole amount. Says I have to close the case first. Is this correct? I did not get to state an amount as the two options were whether I wanted the item or a refund (not how much) and I chose the latter as I wanted the fee back. If I close this I presume I would be unable to open another (same) case for non receipt of the item!? Can't she just use Paypal? Thanks!