Please, contribute in Github to make this FREE plugin for Joomla! better - plugin is very simple, but with huge potential! Needs only small code changes to be updated to support Jooma 3 (currently support only Joomla 2.5).
To work with all Joomla components, mailorder plugin needs working system invoker plugin. So, "System invoker" plugin must be updated to support Joomla 3. Please, help us to make plugin better - to make plugin working in all Joomla! and to share it with Joomla community. You can contribute in Github.