On the customer card, you can set default values for mileage and enter invoice references for each customer.
When you are at the customer card, click the Finance menu. To the left, you may enter if expenses and mileage are billable or not. Here, you may also enter the default distance to the customer. To the right, you may e.g. enter a contact and payment terms, and mark if a discount is given.
You may read more about the features in the product news here.