Does this work?

http://feedback.ebay.com.au/ws/eBayISAPI.dll?LeaveFeedbackShow&useridto=xxxxxxxxxx&item=yyyyyyyyyy&t...

Copy the URL (right click, assuming a PC and not a mobile device), replace the x's and y's with the user ID and item number respectively.

I have done this before where a buyer cancelled after paying (note that I still use the 'legacy' system and not the new format/Seller Hub etc.), I gave them the +1 since they were an honest buyer (unlike serial time wasters which can stay on 0 for all I care).