As a tax prep professional that uses TurboTax, how can I deduct my fee from client’s refund?

Tax prep professionals are not allowed to use TurboTax---it violates the terms of service.  If you are a pro then you are supposed to be using one of the professional versions of the software.

https://proconnect.intuit.com/?s_cid=TT.com_MoreProds-Footer_ProTaxSoftware

Online Terms of Service

https://turbotax.intuit.com/corp/license/online.jsp

See Section B Additional Terms

1.3 No Professional Use.You may not use the Services to prepare tax returns, schedules or worksheets on a professional or commercial basis (i.e., for a preparer's or other fee or consideration).