ummm, pues depende tu versión, pero si tienes la última versión la 1.6 las tablas que tienes que borrar son las 8 siguientes.
'DELETE FROM '._DB_PREFIX_.'order_return';
'DELETE FROM '._DB_PREFIX_.'order_return_detail ';
'DELETE FROM '._DB_PREFIX_.'order_slip ';
'DELETE FROM '._DB_PREFIX_.'order_slip_detail ';
'DELETE FROM '._DB_PREFIX_.'cart_product ';
'DELETE FROM '._DB_PREFIX_.'order_history ';
'DELETE FROM '._DB_PREFIX_.'order_detail ';
'DELETE FROM '._DB_PREFIX_.'orders ';
Si no tienes puesto ningun prefix diferente de ps, puedes lanzar los siguientes querys desde phpmyadmin por ejemplo
DELETE FROM ps_order_return;
DELETE FROM ps_order_return_detail;
DELETE FROM ps_order_slip;
DELETE FROM ps_order_slip_detail;
DELETE FROM ps_cart_product;
DELETE FROM ps_order_history;
DELETE FROM ps_order_detail;
DELETE FROM ps_orders;