You are only required to pay the amount you were billed by eBay.
No further shipping charges can be added*.
Your best bet is to regard this as a failed Transaction, and ask that your purchase be cancelled. That will also trigger a refund, but depending on how the payment was made** there may be a day or so delay.
The seller can cancel "at Buyer Request" without penalty.
Neither you nor eBay can force the seller to ship, so this gets you out of the situation reasonable fast.
You can leave appropriate feedback for 60 days after purchase.
Wait until you see your refunded payment.
The most effective feedback is calm and factual "Demanded more for shipping after payment" for example.
*There is one situation, where the BUYER asks for upgraded and more expensive shipping. The Seller cannot demand more.
** fastest is from a PP balance, but credit card payments are posted directly to the card not to PP.