In the Apps section of our account, install the 'Rounding' application (Premium option required).

A 'Rounding' product is automatically created in your product database. This product is by default not taxed. But you can link it to a tax rate in its product form.

In the app settings, select the rounding rule to apply, and for which payment types amounts will be rounded.

In sales, if the payment method is linked to a rounding rule, the 'Rounding' product will be automatically added to round the total amount of the sale.