If you have an extension to file your 2018 return, you need to pay the balance (if any) that you owe by no later than October 15, 2019, whether you choose to mail it and have the return with your check or money order enclosed postmarked by that deadline, or you can enter a credit/bank account to have the IRS pull your payment from the account, or you can pay on the IRS site:
Your extension gave you extra time to prepare your 2018 tax return documents. It did not give you extra time to pay. If you did not pay your estimated 2018 tax due by April 15, 2019 then when the IRS receives your 2018 return and your payment, they will calculate your penalty/interest and bill you later. TurboTax will not calculate the penalty or interest.
You have until October 15, 2019 to e-file your 2018 tax return. After that, e-filing for 2018 will be shutting down, and a 2018 return will have to be printed, signed and mailed instead. The online software will also be shutting down, so if you are not finished preparing your 2018 return by then, you will have to download desktop software to a full PC or Mac to finish the return.
When you requested your extension you were supposed to pay your estimated tax due. Did you enter the payment you made into your tax return? Go to Federal>Deductions and Credits>Estimates and Other Taxes Paid>Other Income Taxes This will show up on line 17 of your Form 1040
Assuming you are referring to a 2018 tax return and you have taxes owed. You do not have to pay the 2018 taxes owed when you file the 2018 tax return. Just select to pay with a check by mail with a payment voucher, e-file your tax return and not pay by mail.
Since any taxes owed were due and payable on April 15, 2019 you are accruing penalties and interest on the unpaid taxes.
You can pay the taxes owed at any time using this IRS website - https://www.irs.gov/payments
To request a payment plan after the tax return is filed, go to this IRS website - https://www.irs.gov/payments/online-payment-agreement-application