When an order has a promotion applied that will set the shipping costs to 0, the order cannot be created at Mollie. This is because the order amounts don't match.
We need to check if an order has shipping discounts and send them to Mollie when we create a Mollie::Order.
This PR fixes the following issue:
We need to check if an order has shipping discounts and send them to Mollie when we create a
Mollie::Order
.