Tax Rate/Fee Percentage Settings
WP Simple Pay has support for global tax rates or fees that are applied to all forms. You can find this setting by visiting WP Simple Pay → Settings → General → Taxes

Note: An exclusive tax is not included in the invoice subtotal, whereas an inclusive tax is.
To display the tax/fee breakdown on your payment forms, use the Amount Breakdown custom form field.

If you need a bit more flexibility with taxes/fees, we do make a code snippet available that allows you to override the global tax percent or fee and apply a specific rate to a specific payment form. You can find the Custom Tax Rate code snippet by clicking here.
Please note: Custom code snippets are provided as-is and there is no guarantee they will continue to work across multiple versions of WP Simple Pay.