Hi Niels,
Perhaps this page can help you: https://docs.mendix.com/howto/logic-business-rules/working-with-lists-in-a-microflow.
For calculation of totals: https://docs.mendix.com/howto/logic-business-rules/working-with-lists-in-a-microflow#4-calculating-a-total-list-value-using-a-variable-and-a-loop and https://docs.mendix.com/howto/logic-business-rules/working-with-lists-in-a-microflow#5-calculate-a-total-list-value-using-an-aggregate-function.
I think that when you read this page, you will get your answer on how to get the total price for OrderNumber 1.
Kind regards,
Jeffrey
Make a list of the orderlines for which you want to calculate the price. The total price can be calculate with the aggregate list action.