RE the php.ini file.
On some hosts, this will be processed, and the settings respected. On others, it will not be.
This depends on how PHP is installed and configured. Unless this setting is in a php.ini file in the Abantecart distribution, it is NOT an Abantecart bug, but a hosting issue which needs to be addressed by either reconfiguring your hosting account, or getting your host to do so for you.
If you do find a 12KB limit set in the distribution, I would report that as a bug immediately..
David