You have until April 15th to pay it according to my tax lady. The coupon that goes with your payment has it April 15th on it as the due date. Not sure why ADOR would be calling or writing demanding money. Are you sure the letter was legit? I'd take the letter to my tax person and have them respond for you. Mine always handles stuff like that for me.
If they can demand you pay it within 10 days then I should be able to demand they pay mine immediately since I filed my taxes the first of February and haven't gotten my $50 back yet.