Coding in form for a2003

I have an Orders form. It contains fields tied to an Orders table for Invoice
Number, Order Date, Line Total, and Supplier ID, and a subform Order Details
that is connected to an Order Details table that lists in a datasheet Product
ID, Quantity Ordered, Quantity Shipped, Unit Price, Subtotal, and Sales Tax.
I also have a Product table that lists Product ID, Product Name, Unit of
Sale, Unit Price, Taxable (yes/no), Hazardous (yes/no), Supplier ID. I've
created my one-to-many relationships and the Orders form is functioning
properly. But I would like to take it a step further. In the Orders form I
would like to be able to choose the products that were ordered (whether one
or several) for each invoice, have its associated data pull up in the form,
and be able to change field data to allow for price fluctuations. Also,
currently the Subtotal and Sales Tax fields in the Order Details table are
manually entered; is there a way to automatically calculate these? To sum up
I am looking to improve my Orders form to give the user the ability to,
having entered the invoice number, order date, line total, and supplier id,
choose from a combo box the products ordered, have that choice reflect in the
form, update the price as necessary, and move to the next product for that
order. Thanks for any help.

Brian