Great start :))

In the text i would be crystal clear about the supporting evidence you are attaching. For example, as stated in the listing the camera is under warranty a copy of which can be found on their website (and put in the exact full website address if the warranty page).

I would definitely reference the actual paypal transaction number of the original purchase - thats almost the best proof of all.

Also add in the buyer has sought a partial refund for xxx and that the reasons they sought a refund have changed throughout your communication with them.