To display the sum of fields Amount and Freight, place a 3rd text box on
your form, and give it these properties:
Control Source: =Nz([Amount],0) + Nz([Freight],0)
Format: Currrency
The Nz() tells it to treat the value as a zero if it is Null.
The Format tells it to treat the result numerically.
Use General Number here if you don't want dollars.
This text box must not have the same Name as one of the fields, e.g. it
can't be called Amount or Freight.