Dont play games or beat around the bush. Create a pdf file with the emails in chronological order and highlight the important parts.

"The item you are asking about is not part of the original scope and we do not have an agreement after the fact. I apoligize if there was any confusion. Please reference the attached PDF file that shows our email exchanges in chronological order."

Ask if they want to pay you to bring the item in question out and install it. Done.