Kartris 2.0 and onwards are licensed by default with the GNU General Public License v2 (normally abbreviated to GPL v2). This is a free open-source license. Please read the license terms carefully - in particular, the license requires you to ensure that you do not remove code that displays copyright and warranty information to users. So the 'powered by' link at the foot of pages must be retained under this license.
This license permits removal of the 'powered by' link from Kartris sites for the domain or subdomain for which it is purchased. There is no expiry date on the license, so you can use this with future Kartris releases - once your domain is licensed, you can upgrade to a newer Kartris version and continue to use this license.