No, it won't automatically charge you. You would have had to take active measures to pay for it with credit/debit card after it showed you an Order Summary. So you should NOT be charged for it merely by selecting it by mistake. But it does cause a problem that TurboTax Support will need to resolve for you.
Normally, if your return was still in preparation, you would be able to clear and start over and get back to Free Edition in order to upgrade to PLUS. However, since you have already filed, you can no longer clear the return. So if you try to print now, it will indeed charge you for Live Basic. So here's what to do:
Phone TurboTax Support tomorrow. Since you are currently in Live Basic, that product qualifies for phone support. Tell them exactly what happened. Tell them you upgraded too far by mistake after filing and ask them to unlock the 2017 return. It's possible they won't even charge you the 19.99.
The other option is to get the free return transcript from the IRS as I mentioned in my comment above.
TurboTax Support Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Monday-Friday
FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
