I think I have managed to write a payment plugin using the build-in 2 payment options as examples.
However, I'm not too clear on how to install it as plugin. Is there an exmple of a payment plugin that can be installed with Joomla Extension manager available? That would help greatly.
Certainly, all you need is an XML file describing the plugin. Zip up the XML and PHP file together and you should be able to install via Joomla! installer. I can send an example if you need it. Just PM me your email.