Packages are a very helpful tool when it comes to building invoices and using them in proposals (and clients picking their own packages to checkout with). If you are building invoices and are typing line items in each time...STOP and use packages! ;)
To set up packages, you can either do so when creating a proposal after adding the package element.
Or you can set up packages by heading to TEMPLATES >> PACKAGES. Then click ADD PACKAGE to add your package.
Packages can be one single item / add on OR it can be a collection of a whole bunch of things!