You do not need to finish you tax return before 03/01/19.  You can pay for the online edition now to lock in the lower price.  Another benefit of paying for the online edition before completing or filing the tax return is that you will be able to view or print your Complete tax return at any time instead of just being able to view the Form 1040 only.

To pay the TurboTax online account fees by credit card, without completing the 2018 return at this time, 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.