Using products in invoices
To use products in your invoice, you need to have Products plugins activated. 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 dropdown with all products you have in your Redmine. Click on the one(s) you want to add, and it will be added to the invoice, with fields filled automatically. You can of course edit them if needed.