good luck getting through to the IRS on its phone lines. they're swamped and taxpayers frequently don't get through. if you do get through do what they tell you. if you don't get through I would mail back the notice without a check and include a letter of explanation that states the $842 due was before the adjustment for my nontaxable portion of my unemployment compensation. taking that into account the IRS owes me a refund even without me paying the $842.
if you send it, make sure you get proof of delivery.
an amended return would have done no good. by the time the IRS got to it, they probably would have fully processed your original return and issued you the refund.
