it could take the IRS months to bill you for the unpaid amount. meanwhile, late payment penalties are about .5% a month. but if you didn't file your return by 10/17 there's an additional penalty of 4.5 % per month for each month late for up to 5 months. the sooner you pay the less interest on the late payment that will be charged. you can request abatement of the penalties if you can give the iRS a reasonable explanation, in their judgment, of why you paid late but interest can not be abated. you can use their website to make payment
https://www.irs.gov/payments
