take photos of every step of the process. How you pack them, packaging materials, how they look in the box once packed etc, get photos from every angle in case they say they were damaged etc and keep lodgement receipts with a post code and tracking numbers.

Not sure if the photos will ever be any help, but if they claim broken on receipt or SNAD at least you may have something to fall back on.

I'm guessing buyer is going to be a little bit miffed.

But OP, as far as I can see, you have done nothing wrong and have nothing to feel bad about. It seems to me that you have really gone the extra mile to please this buyer, But just make sure to protect yourself in every way possible now.

Chances are, they obviously really want the item, so lets hope the thrill of receiving it leaves them in a good mood, eh?

Best of luck.