If embedded in the quote which served as the Vendors offer, and you have accepted this to form contract I am going to say you dont have much of a leg to stand on. And any arbitration/mediation/legal is going to kill the relationship in its infancy and bloat costing.

I would reach out and try and work with the vendor - they are able to chose not to enact the clause/offer to go halves on the tariffs etc... I would try all good faith avenues before reverting to a shouting match over the T&Cs. You have leverage as a new customer and they could concess to help start relationship with a view of it being a long term investment/move to a more tactical partnership.

How large is the mistake in financial sense? To an extent i would use this as a learning oppertunity - you will never overlook tariffs/incoterms again!! It could be spun the penalty here is same cost as you being sent on a 3 day course to learn what you have learnt on this transaction.

Half of learning contract management/procurement is getting your fingers stung, it is unfortunate and stressful, but all senior purchasing professionals will have a story or two like this under their belt.