Billing mileage and per diem charges on projects

I'm wondering what method is "Best Practice" for service reps to enter the billable mileage they travel on a service trip, and associate that charge automatically to a job. We bill out at $/mile, but we don't compensate the employee (company vehicle), so its not an employee expense. If possible, I don't want to create a sales order, tied to the project, for just that item.

Also struggling with a clean way to handle per diem charges. We pay expenses for employees, and often will charge per diem to customers. How have any of you made that process smooth, so that we don't have to add it at invoice time, or create and update "fake" linked sales orders?