Do everything jellybird said.

I really just came on to second it.

Only reply or write to a seller within ebay messages, if you can.

You've already paid and you have already sent a message to say you won't be paying more postage.

And no, you never do have to pay extra. This seller knows that which is why they have contacted you off ebay messaging. They are trying to take advantage of you. No need to communicate with them any more.

What you need to do now is have a look at your purchase listing and note the estimated date of arrival. If the parcel is not there by that date, open an ebay claim for item not received.

The seller will probably contact you at this point, offering a refund or saying they have posted it out and asking you to close the case. Don't, just don't. Just repeat, you have not received your item.  (If you close a case, you can't reopen it again if you find the seller lied to you.)

After a few days, you will then need to escalate the case to finalise the claim. That should see you get a full refund.

If for any reason, you were not able to make an ebay claim, you could make a paypal claim but never both at same time and always try ebay claim first.

Good luck.