This is not possible as changing the total would change the tax amount.

You could setup billing as the full amount including the tax and then back out the amount of the tax as part of your process for calculating and paying sales tax.

thank you. how would i go about doing that? is there a simple thing already built into timeslips or do i have to create my own system?

You would just bill the flat fee and then, when you go to figure out your sales tax back out the tax amount from the flat fee. I would suggest including a message on the bill that indicates the flat fee includes sales tax. In the event the customer has a sales and use tax audit this will hsow that htey are paying sales tax and will avoid problems for you both. 

The  other alternative is to calculate the flat fee net of the taxes yourself and charge that amount. Then let Timeslips charge the sales tax.