To use products in your invoice, go to invoice's view and click Edit. In the Invoicing lines section, you will notice the Add product field.
When you click on it, you will open a drop-down with all the products that you have registered in your Redmine. Click on the one you want to add to the invoice. You will notice that their fields are filled automatically. You can edit them if needed.
In the deal's edit view there is a Deals items option with a plus icon. After you click on it, you will see a new blank line added. You can add here products for a deal. It is useful if you sell products or customer service.
Adding products here looks almost the same as with invoices. If you click on the New line option - you will be able to fill in all the fields manually. If you type product name in Add product field - line will be filled in automatically. You cannot manually change the sum, as it is calculated from the given deal records.
Later, you can filter deals by Products and Product's category.