Invoices can be made several ways:

Nick finds it easier to make 1 invoice per supplier so you can select the appropriate T&C from the drop down menu to link the invoice to a supplier T&C template you already have saved.

Just remember, invoices can only be 'signed' once. So if you add to the invoice later, best practice is to 'duplicate' the invoice, made the modifications to the invoice, and have the client sign the new invoice for a fresh signature.