Sales tax surcharge rates

HOW DO I ADD A 1% SALES TAX TO A PORTION OF THE AMOUNT OF A SALE?

The 1% sales tax surcharge only applies to the first $5K of an order. The balance of the order is taxed at a lower rate. The amount for this application will be $50.00 whenever it is applied.

I want to make sure the $50.00 goes to the sales tax liability.

When I attempt to insert the aount versus a %, it doesn ot allow me to do that.

 

Thank you

Greg