It's a very fine line when to leave negative feedback. I sell many items a week on ebay. For me it's a business and I treat it as such shipping most items within one day of payment receipt. The risk, as Fred said, is that they will leave you negative feedback and you have a legitimate reason and they only have spite. You can still leave positive feedback with negative comments about the slow shipping. Personally, I would refrain from negative feedback, unless he stops responding to your emails.