If there is a way to pay online I would do that too, but TurboTax wouldn't allow me to pay online because it was after 4/15. I couldn't find an extension/Form 4868 option on the IRS Direct Pay site and I just want to make sure my payment is applied to my 2018 taxes (since I also owe back taxes from 2017 and estimated tax for 2019). Basically my question is, how can I pay an estimate with my extension at this point, either by check or online? I live in Massachusetts.