Two of the trickiest things to design is a professional proposal and a professional invoice. An invoice is something that should be used in every business but usually in start ups or one man companies it is something that is overlooked. It will not only allow for you and your client to keep a record of transaction of payment. It will help your company look more professional, help you keep better records and also know who needs to pay as well as who is late with payment. Setting up an invoice i.