Join against two tables to get single combined result set
In this case, and due Coupon is an specific entry of another table I'd use a UNION.

As a suggestion, I'd try to convert 'Coupon' entry of orders_total, into another product added as another row to the orders table.

SELECT orders_id, products_name as name, cost
FROM orders o 
JOIN  orders_products op on o.orders_id = op.fk_orders_id
WHERE o.orders_id = 1
UNION
SELECT fk_orders_id, name, total as cost
FROM   orders_totals
WHERE fk_orders_id = 1
AND   name = 'coupon'


Results:

| orders_id |   name | cost |
|-----------|--------|------|
|         1 |   Nuts |  100 |
|         1 | coupon |  -10 |


sqlfiddle here