By way of  PS. Rebecca, I do feel your pain as my sister & I were once in a similar situation, where an item we knew was in great condition was sold to a buyer who grouched about postage (she was an international buyer and told us she was shocked she had to pay more than $10 for a handbag to be posted). Then she claimed a fault but wasn't able to show any photos. Demanded a part refund. This was also a buyer who tended to buy a lot of vintage bags for (we suspected) onselling.

At first we were determined not to put up with being what we saw as cheated but after we cooled down, we decided that much as it went against the grain, to take the path of least financial loss. For us, that consisted of making a partial refund rather than paying to get the whole thing back or even worse, have ebay refund, perhaps before the bag was even returned.

You're probably going to be best to keep it cool and professional and just tell this woman that of course if she wants to return it she can for a full refund (with you providing a postage label) or would she prefer a partial refund (to make up for the fault) and to keep the frock.

I think she'll probably still go for the return but you never know unless you ask.