So if you already have that working then you should be able to add a query to your endpoint script which returns all the lines in the order (order_details or something like that?) which can then be used inside the email to the customer.
After the endpoint updates the data record, do a query of the order details tables where order_id = the ID which you already know. Do another query to get the customer details (you just need their email address). Add a Send Email action and build the body of that email to contain everything you want to give them of the order. See this thread for more details on that here: