The apps I make here are free. There are various ways you can donate to support what I’m working on whether it be through Gumroad or the Ubuntu Software Center. But the versions of these apps on the Ubuntu Software Center will not be updated. You can always have the latest versions by installing from the PPA or downloading the individual installers from the PPA packages page.
Set up the PPA by doing this, for example, to install P.S. Notes:
sudo add-apt-repository ppa:thejambi/thejambi
sudo apt-get update && sudo apt-get install psnotes
If the PPA does not work for you, or you want to simply download the .deb packages without using the PPA, you can do that from the ppa package details page.
I recommend using elementary OS to have the best experience with your computer. The PPA will be best supported on the latest version of elementary.