I don't quite follow the " since you are the seller " logic. We are always the seller. But when we made this test user, we explicitly spec'd them with a USA address. We do get dozens of USA payments a week and they never get billed the tax.

From here it looks like the payment processor unilaterally decided to charge Canadian tax to a US buyer, because the test user was located in Canada at the time (via Geo-IP?) and/or the credit card used for the purchase was a CAD credit card from a CAD bank. Just seems mysterious that you say if the tax in is the cart, then it was CB doing it... I don't see why CB would decide it was a "Canadian" buyer when they specified USA as their CB address when signing up. Or is there something else going on?

As for the default settings - see below...

Interesting! Invoice address has always been OFF on our system, but (aside from this one recent anomaly) all US Canadian clients get charged tax, and all US ones do not.

The Tax rate page says its depends on "billing" address, not "invoice" address:
"when a buyer billing addres is in a given Geographic Zone which has Tax Rate(s) assigned to a Tax Rule used by a Product Plan, the corresponding invoice item computes the corresponding tax that is added to the invoice."