When the amount paid is over the amount of the sale, the difference is considered as a tip.
In 'Reports/End of day', in the 'Differences and service charge' section, you can consult all discrepancies on sales (negative or positive).
In some countries, a service charge can be invoiced to the customer. It is often a percentage of the total amount of the sale.
In 'Settings/Users/Access management', enable the right : 'Manage service charge'.
You can set up the service charge rate in 'Settings/General settings/Service charges'.
At the checkout, you can click on the 'Calculate' button to add the service charge automatically in your sale.
You can modify the service charge amount if necessary.