Your mail has to be postmarked the 18th so it is not considered late
Be sure you mail it on the 18th before your Post Office's last postmark time. The cutoff time can vary from post office to post office.
I'd mail it inside at the counter, if it were me, and purchase proof of mailing. Then you'll have the postmark on your receipt.
Just because you indicated in TurboTax that you are paying by check, you can instead pay electronically at the IRS website if you prefer. It will be a free direct debit of your bank account. You can also pay by credit/debit card there, but there is a service fee for those choices, which can be quite hefty.
If you want to view those options, see this website:
<a rel="nofollow" target="_blank" href="https://www.irs.gov/payments">https://www.irs.gov/payments</a>
Also, here's a TurboTax FAQ with some info on paying by check.
<a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/2580011">https://ttlc.intuit.com/questions/2580011</a>