Can I pre-pay for Turbotax online in 2017 - before I file - so I can deduct that tax preparation fee on my 2017 taxes (since it will no longer be a deduction in 2018)?

It's possible to prepay for the 2017 Federal return.  You'd also have to start a State return to prepay the State return (if any.)

However, bear in mind that it's often hard for many people to qualify to deduct the TurboTax fees.   First, one has to be able to itemize deductions on Schedule A.  So the total of all deductions must exceed the standard deduction.    And tax prep expenses are in the category "2% miscellaneous deductions", which means they only count when the total of those miscellaneous deductions is greater than 2% of AGI.   So unless one can itemize and has a lot of miscellaneous deductions (employee job expenses, job search expenses, tax prep fees, investment expenses, etc.), then the TurboTax fees would not count.

That being said, to pay in advance you simply pay for printing privileges, which pays for the return.  Then as a side benefit you can also view/print forms when necessary during preparation as well.   It also locks in the current early-season discounted price before any price increases.

• Log in and  start your return.  Or if already started, open your return "Take me to my return."
• Once you are past the blue-green page and the return is open, click in the left menu column on TAX TOOLS.
• Then choose PRINT CENTER.
• Then choose "Print/save/preview this year's return."
• Choose to PAY NOW, and it will prompt for credit/debit card.   Do not choose to pay later out of a refund.