Maybe a tough call, but personally, and regardless of anything else, I think the seller is out of line if they are being persistent with the request to alter feedback if you have declined.

The neut will really only hurt their business if you also left them low DSRs for anything (1 or 2 star ratings), and even then I wouldn't dispute your right to do so if item location was misrepresented and so forth. Low DSRs can impact Top Rated status and search rankings (especially if more than one low rating is received), but it's wrong IMO to make you feel guilty about that if you feel you left fair and honest FB.

Personally, without knowing the finite details of how the transaction was handled, I think your feedback seems fair, in that it's factual, and their response wouldn't put me off buying from them, either - in fact it's a fairly encouraging response on their behalf.

You have the right to not be pestered, so if you come to a decision and the messages continue, report them to eBay - there'll be a Report this message button at the top of the message in the inbox  (and put them on your blocked bidder list, just in case).