Invoices are made up from one or more line items. You can use custom line items for packages, for discounts (negative line items) or for unusual elements. However the power of invoicing in Sheep is getting the system to work out the total using your price list.
Your price list is managed in Settings > Invoicing & Finance (in Sheep we call each item on your price list a 'service type').
Name and Price are required fields, Category and SubCategory are used for reporting income. Supplier is not currently in use and is reserved for a future version.