I've been advised by a retired lawyer friend not to ignore it as they'll surely issue a tax warrant on it. At the very least it'll show up on our credit report. We could also get our pay garnished.
His advice is to find the statute that covers it. If it applies to us pay the bill. If not contest it based on that. He's 99.9% sure it's in error and we're fine.
Having trouble getting the exact statute as to what they say makes our Florida registered Montana taxable.
I'm retired so I've got nothing but time to annoy them.