Sorry for the lack of clarity. I wasn't inquiring about your personal intentions, as they are none of my business, I was just curious that the item was priced significantly lower than the next listing.
Doesn't appear to be OP's intention. Indeed, tax evasion in any form is unethical, but so is a company retroactively charging someone (regardless of the charge's nature) without prior consent.
I'm 99% sure that you will not be charged sales tax for it. The order was placed before the change, so the order details should remain static. The only way I could see Amazon adding sales tax to the order is if you changed the method of payment. Ultimately, it would be unethical for Amazon to...