I wouldn't go as far as to leave her negative
feedback right now.  I would only resort to that
as a last resort, and chances are since her
account is so young that she really wont care or
will make another one.

Kristina should try to explain all the details to
her again, though it probably will do no good, and
try to contact eBay and Paypal on this matter. 
Since she stated in writing that she would ship
free if the buyer used Buy-It-Now, there is no way
that they can say that she was falsely
advertising.  eBay will have their records of how
the item was purchased along with her own records
that she kept to send to them.

If she still responds as she has, I would tell
eBay that you have tried several times to explain
it to her.  You've tried to send her all the
information you could about it, and that she
obviously is not making any effort to resolve the
issue by talking to you.

If she chooses to give Kristina negative feedback
it will be hard for her to get it removed.  My
sister got neutral feedback on her account for no
reason at all.  She payed for the item as soon as
she got the total and the item *was shipped to
her*.  After it all was done and over with she
asked the seller if they would give her some
feedback if they were happy with the transaction.
 I don't know if the seller accidentally gave her
wrong feedback or did it on purpose, but it says
that she bid on the item and did not pay.  eBay
would not remove the feedback and the seller would
not respond to any of her emails ;/