Shell, Do you VBA?
If so, then you can put calculations into the after update event of the combo box. Something like
'Just for ideas
Private Sub MyComboBox_AfterUpdate()
Select Case Me.MyComboBox
Case "Mow"
Me.TotalPrice = Me.TotalPrice + 25
Case "Mow-Paid"
me.TotalPrice = me.TotalPrice - Me.CostOfLawn
Case "Paid"
Me.TotalPrice = Me.TotalPrice - 25
'More cases etc.
End Select
End Sub
How's that for a start