First, if a turbotax customer support person helped you file an extension, it could only have been for 2020. If you already filed a 2020 return, the extension application will not benefit or harm you in any way, the IRS will ignore it.
It sounds like a simple miscommunication problem, or the support person was poorly trained. I could send a request to the moderators to have your case reviewed so the support person could be re-trained, but I think that's a waste of time at this point since most of the phone support people are seasonal and the person who helped you probably won't have a job by next month anyway.
You should not have been charged extra for this help. If you were charged, then you weren't on the phone with Turbotax, but with a scammer of some kind, or at least an unauthorized support person. Anyone can pay google to be at the top of the search for "turbotax support" -- the best way to get a verified contact number is to use the help request form on the secure web site.
https://support.turbotax.intuit.com/contact/
Second, there is no deadline to file a tax return. There is a 3 year deadline to claim a refund if one is owed to you, but you can file an overdue tax return even after that date, and it would always be recommended to do so. The IRS has 3 years to audit you from the date you file, so if you never file, the IRS can audit you forever. And if you owe tax, there are late filing and late payment penalties piling up. However, even though you can file tax returns older than 3 years, Turbotax discontinues their product support after 3 years, so if you wait too long, you will have to find a professional with older software, or download the forms and instructions from the IRS web site and prepare your return by hand.
Third, if you are ready to file a 2018 tax return and want to use Turbotax, you have to purchase Turbotax for 2018 to install on your own computer. Turbotax Online can only prepare the current year's return (2020).
https://turbotax.intuit.com/personal-taxes/past-years-products/
