Leaving Feedback.

I have been trying and trying to leave positive feedback to a recent buyer for a parts sale.
When i copy & paste the URL of the part that sold on the feedback page, where it's supposed to go, then hit the leave feedback button, it tells me it's an invalid URL, and paste a valid URL to leave the feedback.
So what the hell is going on now?
I'd like to do the proper thing, and leave positive feedback to my buyer, but i can't figure out computers when they act up.
I call mine HAL, like in the 2001 A Space Odyssey movie, a computer that does what it want's.
Any thoughts?