So "overall a positive transaction" but you leave Neutral Feedback. "Confusion over postage" On who's part was the confusion?
Feedback can be detrimental to sellers and it is often better not to leave feedback at all, rather than what would appear to be an unjustified Neutral. In any event harrassment is also unjustified.
Contact eBay Customer Service. Live Chat or via telephone would be better than the standard messaging.
Perhaps have screen-caps of the harassment, in case the seller deletes his comments / messages.
Was the confusion over postage based on the estimated delivery dates that ebay inserts, or over something that was within the seller's control? (did they post it within their handling time? ) or did they promise something they did not come through on?
Have they been harassing you within a message/s or have they sent you a feedback revision request?