Improve line item API documentation

Hi,
Are there any thorough tutorials out there on how to use line items in Übercart 2-6.x? I have found a few, but none of them really explain what is going on and what needs to happen for certain things to occur. For example, my issue is that I need to get a variable that is in my cart-preview case for the line item to also appear in the order pane and the invoice. The line item itself is there, but it doesn't show any cost. I'm sure I've just gone slightly off somewhere and if someone could help, that would be great. I tried to use:

    case 'load':
      $lines[] = array(
        'id' => 'fastpris',
        'title' => t('Fast pris'),
        'amount' => uc_fastpris_get_cost($arg1),
        );
        return $lines;

and then:

function uc_fastpris_get_cost($arg1){
  $fptotal = 0;
  foreach ($arg1->items as $item) {
    $arg1->type = node_load($item->nid)->type;
    if($item->type == 'fast_pris_items'){
      $price_info = array(
        'price' => $item->price,
        'qty' => ($item->qty) ? $item->qty : 1,
      );
      $context['subject'] = array(
        'cart_item' => $item,
        'node' => node_load($item->nid),
      );
      $total = uc_price($price_info, $context);
      $fptotal += $total;
    }
  }
  return $fptotal;

  $order = new stdClass();
  $order->products = $arg1;

  $context = array(
    'revision' => 'altered',
    'type' => 'line_item',
    'subject' => array(
      'order' => $order,
      'line_item' => $line_item,
    ),
  );
}

But there is no value in the order pane for the line item Fast Pris.