Access Database

i want to design a invoice form in access database,but have few problems (1) how can i enter multiple orders under one invoice number. the invoice number is auto-number. (2) when i apply some formula the result is not stored in table, like i calculate profit so [income – exp = profit] but in form it show me the result but not stored in table. i checked the field name are same.
thanx in advance,
cheers.