Hi,
I think replacing this line would do the trick:
' . (osc_item_is_premium(osc_item_id()) ? ' | <a href="' . osc_route_url('payment-premium', array('itemId' => osc_item_id())) . '" >' . __("Destacar", 'payment') . '</a>' : '') . '
with
' . (!osc_item_is_premium(osc_item_id()) ? ' | <a href="' . osc_route_url('payment-premium', array('itemId' => osc_item_id())) . '" >' . __("Destacar", 'payment') . '</a>' : ' | ' . _e('Already premium!', 'payment')) . '
Regards