Solved
It seems I can only use a Check or apply for payments.
If you have already paid your taxes using one of the IRS's official payment processors outside of TurboTax (or you plan to make the payment using their site), then select "pay by check" and you can just ignore the payment voucher that prints up. Selecting "pay by check" doesn't obligate you to pay by check, it simply produces the payment voucher for you to use. You are free to then make the payment another way.
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