What did you buy for 79.99? Did you purchase 2017 desktop Deluxe at 79.99? Or did you buy 2018 online Premier at 79.99? If you bought a 2018 online product, then the 2017 return should no longer be locked if it's in the same account. If you purchased desktop software, then the online 2017 file will still be locked, but I'll tell you what to do below.
Online TurboTax will not know that you purchased desktop software, since they are not linked. Normally the way to automatically unlock a 2017 online return is to purchase an online 2018 product in PLUS (19.99) or a higher edition.
Since you likely bought the 2017 Deluxe desktop software, I'd phone TurboTax Support. Hopefully, they will just unlock the 2017 return so you can download the *.tax2017 data file and finish it in your product. I don't think it's right to make you pay for 2018 PLUS or higher edition on top of that 79.99 Deluxe just to get the 2017 tax file, but you'll have to see what they say.
You can use the form at the link below to reach Support. It may schedule a call back, or it may provide a number. Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Monday-Friday.
Where the form asks what product you are using, be sure in the dropdown menu to indicate "CD/download" and the proper operating system, then indicate which Edition.
The 2017 return cannot be efiled. It has to be printed and mailed. If you have both Federal and state returns, be sure to mail them separately to the respective tax agency. It's a good idea to mail by a method that has tracking such as certified return receipt, priority mail, etc.