It was a bad customer experience - of course her response is negative!
eBay have designed it this way, but many users probably aren't aware of the consequences of leaving feedback.
You'd write a complaint card in a hotel with nary a second thought and no ill consequences. To be sued for what appears to be the same thing online is at best a gross overreaction.
When you keep in mind the effect bad feedback has on a seller, you really should stay your hand unless you feel they were really out to screw you (or you're just plain vindictive).
It's a buck in shipping. I'd pay it, blast them an email asking WTF, maybe leave a neutral at worst. Bad feedback is reserved for fraud, IMHO.
Then again, the seller in this case appears to be a litigious asshat, so fire away.
As someone who has never used ebay, this way of thinking sounds ridiculous. I can't imagine that this is what the average persons thinks negative feedback means (it means "my experience was less than satisfactory" pretty much everywhere else) and this sounds like an enormous problem with ebay.
My point was just that novice buyers have no conception that 'bad' doesn't mean what it means in the rest of the world. Maybe eBay need another level -- "Vindictive", say? ;-)