If you are shipping products, including a packing slip with the shipment provides clear communication to your customers. Indicate what items are included as well as any items that were ordered but not shipped. Not only does a packing slip inform customers, it can also help reduce mistakes before shipping. This is intended as an addition to a sales invoice.
Most businesses use the same sorts of information on invoices: contact information, sequential invoice numbers, etc. Because of the unique nature of freelance work compared to other businesses, especially for freelance invoices, it should be adapted to reflect these differences.