Gracias por la ayuda me hiciste ver algo que no veía, lo he logrado cambiando esto:
Código:
$query = mysql_query ( "SELECT * FROM mcb_invoice_items, mcb_invoice_item_amounts WHERE invoice_item_custom_1 = 'aprobado' ");
por esto:
Código:
$query = mysql_query ( "SELECT * FROM mcb_invoice_items, mcb_invoice_item_amounts WHERE mcb_invoice_items.invoice_item_id = mcb_invoice_item_amounts.invoice_item_id and invoice_item_custom_1 = 'aprobado' ");
espero le sirva a alguien más