If you have tax due, it was supposed to be paid by April 15, 2018. Your extension did not give you extra time to pay your tax, it only gave you extra time to get your documents in order and prepare your tax return. And the deadline for a return with an extension is October 15, 2019. After that, online preparation and e-filing will shut down for 2018 returns. If you miss the e-filing deadline, you will need to purchase and download desktop software to complete your return, which will then have to be printed, signed and mailed.
If you owe tax due, the penalties and interest will be calculated by the IRS based on how much you owe and when they receive your payment. TurboTax will not calculate it. The IRS will bill you later.
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
In addition to the penalties and fees on the taxes due noted above, there will be a fee for filing late. Call then IRS now. They are increadibly helpful in workin out a plan ifyou talk to them before something becomes an issue.