It doesn't appear to me that those fields are enough to determine labor.  You
say that the staff are not paid hourly so just exactly how do you charge for
labor?  In other words, if you had to charge a customer with nothing but a pen
and paper as your tools, what would you do?

A database might not be the best way to do this if you truly only have three
fields.  From a business perspective you could charge an hourly rate even if
employees are not paid that way.

If you look at the Northwind database and see how they handle purchase
orders and invoices you could model that concept to do tbl_workorders, and
tbl_repairorders using a transition table to link the two.

To do totals you need to write a query from your tables that sum up the
repairorder totals for a specific customer/job...you could also do this using
the report wizard and experiment with grouping and sums on your cost field.