Each invoice can contain one or more items.
Each Product can be in many invoices.
You can't have a many to many relationship.
You need an aditional table that links the transaction each product, quantity etc, with invoice details.
Each transaction has one customer and one invoice, but possibly more that one product.
The main input form is based on a query that contains invoice, customer details. The product details are a subform that is based on a query linking the invoice with the product (transaction table).
Hope this is clear - just got back form the bar!