events happening in one table effecting another

hello,
im using access 2003 and vb to create a db, how do get an event in one one table to influence an event in another?

ill explain my db a little further - im designing a db to replace a manual record keeping and invoice system.
i have a company table which holds info about the company and whether they get a discount. Im using a checkbox on the company table to show whether they have a discount.
when i come to the calculate the total price on the invoice tables' form i need to take into account whether they have a discount.
how do i get the code from the invoice form's to look up the discount check box on the company table/form??

if you need more info just ask

:confused: :confused: :confused: :confused:
please help :)
thank you