the box is greyed out and only lets me use today's date for some reason.
Don't worry about that. You can say you will mail a check and then go online to pay.
Pay the IRS directly athttps://www.irs.gov/payments
I assume the payment type is "extension" in the online federal payment system, since this is an automatic extension per the natural disaster policy?
No, not extension. That's if you filed an extension back in April and making a payment for it. This is just the Balance Due payment.
if i already scheduled it as an extension, do i need to cancel and recategorize as a balance due?
actually i just called the irs to double check, and they said extension is okay:) phew!