Hubertus,
You need to add a non-persistent object DeliveryProduct and a rule to object Delivery with the following actions:
FIND Product WHERE EXISTS OrderDetails WHERE (OrderDetails.Order.Devlivery = Delivery AND OrderDetails.Product = Product)
FOR EACH Product CREATE DeliveryProduct WITH DeliveryProduct.Delivery = Delivery, DeliveryProduct.CeilingTotal = CEILING(SUM OrderDetails.Total WHERE (OrderDetails.Order.Devlivery = Delivery AND OrderDetails.Product = Product) / 1000)
Delivery.CeilingTotal = SUM DeliveryProduct.CeilingTotal WHERE (DeliveryProduct.Delivery = Delivery)