Well, if you are using the Deluxe online edition for the 2018 tax return then that is the edition you need to pay for to gain access to the prior year online tax returns. The Deluxe edition is only used if the tax data you have entered is not supported by the Free edition. Upgrading to Plus from the Free edition only gives you access to prior year returns and live TurboTax phone support. Plus only supports the same tax data as the Free edition.
To pay the TurboTax online account fees by credit card so you can access your prior year online tax returns, click on Tax Tools on the left side of the online program screen. Then click on Print Center. Then click on Print, save or preview this year's return. On the next page, to pay by credit card, click Pay Now. On the next screen just click on the Continue button. The next screen will show a review of your Fees, click on Continue. Complete the next screen to pay by credit card and click Continue.
You can contact TurboTax support although they may only give you the same explanation and require you to pay for the Deluxe edition so you can access the prior year online returns.
Use this website to contact TurboTax support during business hours - https://support.turbotax.intuit.com/contact/
Support can also be reached by messaging them on these pages https://www.facebook.com/turbotax/ and https://twitter.com/TeamTurboTax
