There seems to be a typo in the lib/class.order_maker.php file at line 271:

    $discount = $item->get_unit_dscount() + $item->get_unit_price()*$val*-1;

Should be (notice the extra 'i' in 'discount'):

    $discount = $item->get_unit_discount() + $item->get_unit_price()*$val*-1;

The current situation causes a fatal error when applying a percentual discount
to the order:

Fatal error: Call to undefined method cg_ecomm_cartitem::get_unit_dscount()
in
/home/tandrup/domains/tandrup.nl/public_html/modules/Orders/lib/class.order_maker.php
on line 271