You could create a charge like this by modifying the basic coupon code discount that is covered in the Creating a Coupon System Solution Recipe:
ecart_coupons_sr.pdf
This can be found in the eCart support page under Solution Recipes:
ecart/
You would just need to modify it so that you filter a recordset based on the zip the user enters, then store that rate into a session variable. From that point it is the same as the coupon except that you are creating a tax or charge rule instead of a discount.