We strongly advise AGAINST using online bill pay for your taxes. The banks still mail us a physical check when you submit your online request. The companies that print and mail checks for the banks use a different class of mail that DOES NOT receive a USPS postmark. If a payment arrives after the due date, the lack of USPS postmark will result in it being late and you would be liable for interest on those payments received after the due date.
We have prepared this primer on the perils of using online bill pay.