Was your top advertised as "Make an offer".
If not and she just messaged you, asking for a discount, then that would ring a slight warning bell for me. I think it indicates someone who only sort of wants the item. They wouldn't be happy paying full price. And when you lower the price for them, that confirms it to them that it was overpriced.
Did the buyer actually pay extra for express post? The reason I ask is because you say you would not have taken the cheaper price offered if you had known that. I get the feeling she got that as an added free extra. You would have been within your rights to say no, the price included the postage as advertised and express post would cost an extra however many dollars.
Block her from buying from you again and just do as digital ghost says-compose a message offering her a return (at her own expense) and stick to it. At the moment she thinks you're a pushover. There are some people like this who, once they get their own way, don't feel grateful, it just fuels their sense of entitlement.
Just be polite, but firm and as DG says, stick to your script.
