Do you always bill hourly? You can just set up columns that have Hrs, Rate, and Total (and of course a description column to detail what the service was). Or if you sometimes do hourly and sometimes flat rate, just have a Unit of Measure column, then have a Qty. and a Unit Price, and Total.
So if you are charging by the hour it's Qty (number of hours), UOM (hours), Unit Price ($ per hour) and Total (Qty x Unit Price). If you are doing a flat rate it's Qty (1), UOM (ea.), and both the unit price and total are the same number (unless you did more than one vehicle at that flat rate, then you would change the qty, etc.