Cash is always checked

One thing I've discovered is that the 'cash' payment method is always selected for the module. This is a result of the way Drupal saves checkbox arrays - if the value matches the index, then the box is checked, and if the value is zero, the box is not checaked. The problem here however is that the index for the 'cash' checkbox zero, so whether it's checked or not, the value is zero, and since the value matches the key, Drupal checks the box.

To fix this will require changing the key for the cash checkbox to something else - maybe 'cash'! That would solve the problem right away.