I filed an extension and I paid the money I thought I owed as I was not expecting to file my return before April 15th. I have since discovered, however, that I underpaid what i am going to owe. How do I go about paying the remainder before April 15th without incurring a penalty/interest as I would if I waited until I filed my return? Can I just file a second extension with the remaining payment? Is there a form to use just to send in a payment in alone? If I file my actual return before April 15th will I run into any problems since I already asked for an extension?
Thanks in advance for any guidance.