I filed Tax extension & said I'd pay $800 Apr 15. When I found I could pay in full TurboTax never said to just pay remainder. IRS took extra $800. How do I get it back?

[Critter]

Ok ... so you filed the extension and made a payment.   Then you completed the return and failed to verify the payment with the extension was accounted for in the program prior to sending it  so you accidentally authorized the IRS to take the entire payment which they did ... well the IRS will eventually see that you overpaid and will refund the excess ... expect this to take up to 6 weeks or more.