Forms are not setup as I suggested. Customer form should not be a subform. It should be the main form. Then Order form is on Customer form, then OrderLine form is on Order form. Three levels of forms, synchronized at each level with Master/Child properties of the subform containers. Anything else will require code or manually opening and closing independent data entry forms.
1. set the Default View property to something other than Continuous
2. fix the form arrangement before addressing this issue
3. OrderLine form in Datasheet view click the Totals (looks like sigma) button on the ribbon, otherwise will need code