The 'Extra charge' application allows you to apply a surcharge in a sale calculated from the total amount of the sale.
It allows you to apply an additional tax or to manage a service charge.

In the 'Apps' section of your account, install the 'Extra-charge' application.

The extra charge is associated with a product of your account which will have the name of the tax or the surcharge to be applied.
It is therefore necessary to create first a product in the 'Products' section of your account and enter the product identifier (ID number) in the application parameters.
The tax rate associated with this product is generally 0% (to be set in the product file).

In the application settings, indicate the rate (e.g. 1.5 for 1.5%), as well as the base used to calculate the surcharge (either the total sale amount excluding or including taxes).
It is possible to round up the amount of the calculated surcharge.

In each sale, you will find an 'Extra charge' button (which can be renamed with a CSS rule), which will apply the extra charge to the sale.
If the sale is modified, just click on the 'Extra charge' button again and the associated line in the sale will be updated.