1. Open your shop in ShopFactory

2. Click -> Central -> Taxes

3. Enter the tax label in Name of Tax Like VAT

Enter the Tax Rate 17.5%

Choose "all existing and new products"

Then choose United States in Where does the tax apply and click Next

4. The next window will display 

5. Untick the box "prices include this tax"  and click Finish.

This ensures your prices do not include tax and that Tax is only added at Checkout page.