Hi Daniel,
Took a look, and believe I know the problem, from the confirmation page url. In www/administrator/components/com_virtuemart/class/ps_checkout.php inside function list_payment_methods is this logic:
<?php
if( $VM_CHECKOUT_MODULES['CHECK_OUT_GET_PAYMENT_METHOD']['order'] != $VM_CHECKOUT_MODULES['CHECK_OUT_GET_FINAL_CONFIRMATION']['order'] ) {
if ($count <= 1 && $cc_payments==false) {
vmRedirect($sess->url(SECUREURL.basename($_SERVER['PHP_SELF'])."?page=checkout.index&payment_method_id=$first_payment_method_id&ship_to_info_id=$ship_to_info_id&shipping_rate_id=".urlencode($shipping_rate_id)."&checkout_stage=".$VM_CHECKOUT_MODULES['CHECK_OUT_GET_FINAL_CONFIRMATION']['order'], false, false ),"");
}
elseif( isset($order_total) && $order_total <= 0.00 ) {
// In case the order total is less than or equal zero, we don't need a payment method
vmRedirect($sess->url(SECUREURL.basename($_SERVER['PHP_SELF'])."?page=checkout.index&ship_to_info_id=$ship_to_info_id&shipping_rate_id=".urlencode($shipping_rate_id)."&checkout_stage=".$VM_CHECKOUT_MODULES['CHECK_OUT_GET_FINAL_CONFIRMATION']['order'], false, false),"");
}
}
?>
If you can read that, it says, if there is only 1 payment method, then choose it and move to the confirmation page, otherwise if the total is <= 0, then dont choose any payment method and move to the confirmation page. I think the total <= 0 should be checked before checking to see if there is just one payment method, so try switching it around like this
<?php
if( $VM_CHECKOUT_MODULES['CHECK_OUT_GET_PAYMENT_METHOD']['order'] != $VM_CHECKOUT_MODULES['CHECK_OUT_GET_FINAL_CONFIRMATION']['order'] ) {
if( isset($order_total) && $order_total <= 0.00 ) {
// In case the order total is less than or equal zero, we don't need a payment method
vmRedirect($sess->url(SECUREURL.basename($_SERVER['PHP_SELF'])."?page=checkout.index&ship_to_info_id=$ship_to_info_id&shipping_rate_id=".urlencode($shipping_rate_id)."&checkout_stage=".$VM_CHECKOUT_MODULES['CHECK_OUT_GET_FINAL_CONFIRMATION']['order'], false, false),"");
}
elseif ($count <= 1 && $cc_payments==false) {
vmRedirect($sess->url(SECUREURL.basename($_SERVER['PHP_SELF'])."?page=checkout.index&payment_method_id=$first_payment_method_id&ship_to_info_id=$ship_to_info_id&shipping_rate_id=".urlencode($shipping_rate_id)."&checkout_stage=".$VM_CHECKOUT_MODULES['CHECK_OUT_GET_FINAL_CONFIRMATION']['order'], false, false ),"");
}
}
?>
I believe that should fix the problem.