Fight it.
Name and reference the messages where you advise her for the shipping arrangements, name and reference the messages in which you offer tracking (noting she doesn't take this up), and then post them a link to Royal Mail's website with the price.
As long as you fight them, and you can prove that she declined the tracked shipping you offered, I'm sure they'll change their mind. I'd probably add in that you have followed all of Ebaay's rules, are dismayed at their treatment of sellers, etc, etc.
Then close your account with Ebay (or least, never sell on Ebay again).