Ship it the cheaper way and refund the difference. 
 It sounds like the buyer is jerking you around.

Unfortunately, I'm guessing that no matter what 
you do, you are going to get a negative feedback. 
 If that is something you desperately want to 
avoid, contact Ebay and tell them your situation 
("Buyer won't pay full price and is being rude 
about it.  You want to cancel the transaction and 
avoid negative feedback.  Can this be done?")

If you DO ship the cel, cover your butt.  Pay a 
little extra for some kind of tracking so that if 
the buyer claims it didn't arrive, you have proof 
to the contrary.  If they ding you with negative 
feedback for failure to deliver, you can probably 
scan the tracking receipt, send it to Ebay, and 
have them remove the feedback as fraudulent.

Just some worst case scenarios to ponder.  :-(

Sorry.

JP