Hello

That is a very good question.

You have to make fields like price, tax, discount, address, phone, etc all static fields (i.e. text, number, phone fields). The big question is should the product also be static? Will you ever delete products from your tables?

The next thing to consider is how to enter all the information automagically.

You probably have a reference field to Products in your Invoice Line Item table. In the Trigger after update you can add:

price := Products.price

So when you select a Product it will copy the price of the Product to the price field in Invoice.

If you have a dashboard, then the button you use to create the invoice would do the same thing, but now you can check for duplication before creating invoices or line items.