Using A Calculation Field
If you would like to implement tax you can use the Product Field configured as a Calculation field type. Then configure the formula for the calculation to calculate the appropriate tax.
You would do this by adding up the value of all of the Pricing Fields that exist on your form, and then multiply them by the appropriate tax rate.
Example:
{Product Name (Price):1.2} * {Product Name (Quantity):1.3} + {Product Name (Price):2.2} * {Product Name (Quantity):2.3} * 0.1
The above is just an example of a basic calculation formula to calculate a 10% surcharge over two single products. If you’re new to calculations, please check the Using Calculations documentation page.
If tax should only be applied in certain situations, you can then use conditional logic to show or hide that field based on the conditions that must exist in order for the tax to be applied.
Third Party Possibilities
Here are a couple of third party options to make this process easier, or possibly suite more use cases:
GravityWiz Subtotal Merge Tag (for Calculations)
Snippet that offers a merge tag called {subtotal}, for calculating the subtotal of all your product and option fields without having to manually enter these yourself. A 10% tax could then be calculated like so: {subtotal} * 0.1
Gravity Perks eCommerce Fields
Adds pricing fields like Discount, Tax, and Subtotal. Also adds special merge tags for including the subtotal, coupon and discount amounts in calculation-enabled fields.