The IRS expects you to pay what you owe by April 18. The extension is only for filing the return, not for paying. If you pay after the deadline, they will charge you penalty and interest.
One thing you can do to avoid the penalties is to go ahead and file and sign up for an installment agreement, where you'll pay an amount every month. A payment plan, in other words.
You can do this in TurboTax in the File section, when TurboTax asks How do you want to pay your federal taxes? You'll check the box that says I want to request an installment payment plan. This will relieve you from any penalties, but interest will continue to be charged at (currently) 4% per year. In contrast, if you file an extension, the late payment penalty is .5% of tax owed per month, with a max of 25%.
Installment agreements do have user fees attached, depending on the payment arrangement (see those here https://www.irs.gov/taxtopics/tc202.html). You can also select a "full payment plan" which gives you 120 days to pay your total amount, with no user fees, but penalties & interest do apply. The above link discusses this as well.
