In TurboTax Self-employed you can enter factoring costs as follows:
Almost any of these, Interest Payments, Legal & Professional, Other Miscellaneous Expenses, has blank lines that you can type in your own description "Factoring", so you'll know what it is when you look at it later.
What about turbo Tax for S-corp? Where do I put the Factor Fees?
Factoring Fees are reported for an S-Corp in a similar category as Schedule C.
In TurboTax Business, this deduction is entered under Federal Taxes >> Deductions >> All other expenses, using a custom description for the amount.
Note: If you enter a general business expense in the wrong category, it will not affect your taxes. Just be sure you don't enter the same expense twice.