Add the demo project in twice.
Select the option for the first one: this is a demo can be downloaded for free.
Then for the second one set the price to $5.
Then people can choose to get the demo for free, or pay $5 for it - in which case you would have to manually send them a download key for the full price project. However, if you're going to do that, you may as well just change the price to $5 because everyone will most likely choose that option and you're creating extra work for yourself.