Do I have to use the Software Center to install a non-free package?
- Can I make a purchase using apt-get?
- How about if already made the purchase and I want to re-install the software. Can that be done with apt-get?
Software Centre performs 2 distinct functions. You can look up applications, and then install or remove then. But, it is also invoked by default (as a GUI method) to install .deb (or Debian) files.
So NO, you could use some other method (there are many) or install a .deb package in a non-GUI manner.
Typically the 'software key' is separate from the software. You only need a PPA to apt-get from. You may then type in, install, or retrieve the software license (or key) to fully enable the software.
Some software in partially functioned without a license, others are fully functional for 30 or 60 days.
That simply depends on whether there is a PPA, and if the license is separate.