Go to Settings > Templates. The first Items tab is the base for building all of your packages. Create each specific item that you offer in your business. This could be anything from the different sessions or additional fees/items that your business offers ie. travel expenses, additional photographer, album prints.
To add a new item, hit the New Item button and fill in the details that apply to the item you are adding.
If you'd rather just build off of the items that we have already added, you can easily edit or duplicate each item by hitting the menu on the right of each item.
Alternatively, you can add new invoice items when creating an invoice from within a project.