I filed extension 2016 -Trying to pay these taxes & wanted to efile, but you don't give credit for what I already paid, so I can't. How to cancel efile payment?

To enter the taxes paid when you requested your extension go to Federal>Deductions and Credits>Estimates and Other Taxes Paid>Other Income Taxes

Further--you are not charged an extra fee to e-file.  The fee is for the use of the software to prepare your tax return, and it is the same whether you e-file or print and mail the return.

Update Oct 2020:  I searched in Deductions section and was unable to find it when the tool walks you through all deductions, BUT if you select Show Topic List at the top right, scroll to Deductions & Credits > Estimates & Other Taxes Paid > Federal, State and Local Estimates and Federal Taxes Paid and select, it will open window to enter multiple options of which one is that you already sent money with your extension. Enter the amount and the system will estimate how much you still owe, or the amount of refund that you can expect.