There's still some things that aren't making sense with my taxes that I need to work on, but I would like to pay the underpayment amount so I don't have more fees.

[pk]

 For IRS, go to www.irs.gov  and loo for  payments --EFTPS -- use one of the options to make a payment for  tax year 2018 ( credit card, debit card, direct debit etc. )

Ditto for your State

[fanfare]

You don';t have to sign up with EFTPS, you can make a payment directly on the IRS website.immediately.