Why can't you send in an extra payment? You can pay anytime directly at https://www.irs.gov/payments
But anyway if Turbo Tax calculates an underpayment penalty next year you can probably eliminate it (or reduce it) by filling out form 2210 and putting down when you received your income.
Here, look at 2210 https://www.irs.gov/pub/irs-pdf/f2210.pdf
and instructions https://www.irs.gov/pub/irs-pdf/i2210.pdf
