I'm still learning about the cart so have a thought.
While not ideal, a work-around may be to create a drop down or radio box indicating the fee for whatever the free form line item is and they select the option and then enter the text (the text box and the radio button option would both appear though even though they may not be used.
For example, have the first radio box indicate "no text" at $0 price. Then the next "one line of custom text" at $10 then "two lines of custom text" at $18
Then under that, put in two text fields for first line/second line. You can limit the number of characters.
The down side is the customer can enter the text but click $0 on the price. In that case you would have to modify the option in the order and request payment for the additional text.