If it is showing unpaid, (if the customer paid cash or via bank) just select the item in your sold list and click on it, up the top - edit - mark as paid and posted, then you should be able to leave feedback.

I'm getting the same f/b glitch that I had a few days ago - box comes up saying "item does not exist or you have already left f/b". I've tried going through f/b forum and every other available option but still no luck. I think I'll leave it for a few hours and try again this evening X-(

I was trying to leave a positive feedback to a customer who bought 3 items for me, but eBay's feedback sent me to invoice... I am wanting to change this, as the invoice has already been paid, and I wanted to put positive feedback to customer... is there a way to fix this?

try using the feedback link from your Accounts Tab on your my eBay page