Thank you very much, that certainly helped me a lot! After about 5 hours work, the invoice system works perfectly!

A little background.

I have a database to record newspaper deliveries. To enter a new order, you input the customer details etc, and then there are 7 tick boxes in the table, one for each day. The user will tick the box to indicate the customer wants the paper delivered that day. So for example if the customer wants the Guardian mon-fri you would put in their details, then the newspaper title then tick the monday, tuesday, wednesday,...