Do the calculation in a microflow and feed the result to the list. Now the list will allow aggregation.
Please note that associating the Service to the Order, has the disadvantage that you can no longer change Service.Price once one Order has been placed since your Order's customer would not like that. Once an order is placed, the price is fixed. Solution to this problem is storing the price as an attribute in the Order.
If you know how to identify “all orders I just made”, you can use OQL to calculate total across services and orders. However, from application design standpoint you should follow Tim’s suggestion and put total price of an order into the order entity.