Sometimes it could really be an honest mistake by the seller by not having the shipping calculator set up properly. Is the seller fairly new or experienced?
Then it kinda depends on whether or not you want this item badly, and whether or not it's offered for a comparable price by other sellers. If this is a one-of-a-kind item that you really want, then sucking it up and paying for the extra shipping might be your only option. Though I would leave appropriate feedback saying that the seller miscalculated shipping and did not honour the shipping rate the listing offered. I do think that sellers are actually not supposed to ask for more shipping money than what was stated in the listing though, so hopefully someone else can provide more info on it.
If you don't really need the item badly, then I'd tell the seller that you'd like to cancel the order as you find the item to be more expensive than expected with the extra shipping added. If you already paid, ask them to refund you, and then go from there.
PS. I found it funny how since the mod has removed the message your seller sent you, your post became hard to interpret without the full context. But I saw your post before it was edited so I knew what the content was (seller messaged the buyer saying that the shipping was a few more dollars than listed and asked the buyer to pay more, in a rather blunt manner). Hopefully my answer is helpful to you.
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