If you want to calculate the orderlines for just one order, you will need to use your current order as the parameter for your retrieve list action. Like this:
[orderline_order = $order], where $order is your current order.
Or use ‘retrieve by association’ from your current order, but note that this will also retrieve orderlines that have not yet been committed to the database.
Based on your explanation, you want to get the sum from a sub-set of objects. this can be achieved by