I'm not sure you're asking for advice or just describing your experience but I'll comment.
LEAVE the feedback and report the seller for trying to extort a positive feedback!! Sellers can't bribe buyers to leave positive (or change what they've left)! Clearly this seller knew what she was peddling and wants to buy her way out.
Additionally, for a SNAD return, you should have received a prepaid shipping label or refund of your return shipping. You should have been made whole.
ETA: Here's the policy of which she's completely in violation. (It's not just buyers who use feedback extortion!)
http://pages.ebay.com/help/policies/feedback-extortion.html
The seller can't require a buyer to leave positive Feedback, specific detailed seller ratings, or revise existing Feedback in exchange for a number of things, including, but not limited to:
- Receiving the purchased item
- Issuing a full or partial refund
- Monetary compensation
- Additional items
- Other goods and services not included in the original listing