There is an error of some sort with the eway plugin, as when it is enabled, pages fail to load when that is available as an option for payment.
There was an error on installation - reported in Joomla - a missing lang file (wasn't in the installation package): en-GB.os_eway.ini
However, when enabled, the page load fails - no error log - just a white page. Other payment plugins enabled (without eway) works.
However, looking at the form post fields, it seems as Joom Donate may not meet PCI DSS compliance standards, and it looks like the post is directed to a Joomla URL, and not directly to the cc processor (ie Direct Post method); which is the only way that it can run according to financial industry standards on a non PCI compliant server. I tested this theory using the 'Authorise.Net' payment plugin. Hoping that if the eway plugin is working it may not also have this flaw.