He sent you an invoice by Registered Letter?
This guy is weird.
And no he can't add anything to his advertised shipping cost. It is firm once he puts it on his listing.
Personally, I'd send a note . "Thank you for your correspondence regarding payment for lot #xxxxxxxxxxxxx, Title, which I ordered on December 28, 2014.
I have recieved an invoice from eBay for $X,xx, which was the amount I understood would be the total price when I made the order. Since you are apparently unwilling to sell at your advertised price, please send me a Mutual Cancellation which will free both of us from a transaction which cannot be completed. "
If he does, agree immediately and leave appropriate feedback.
Don't overlook the Detailed Seller Ratings. The lowest Rating is ONE Star.
BTW, I'm thinking he is trying to move the transaction 'off-eBay'. Which is pretty dumb, because he does not realize that Paypal's Buyer Protection is just as strong as eBay's.
So you have a seller who is sneaky and dumb.
Move on.