ShipStation error: "Bad Request: request.serviceCode: The serviceCode field is required."

When generating labels, Return Magic usually retrieves a list of rates to select the cheapest rate possible. With some carriers however, Shipstation will not send back rates. In that case, Shipstation suggests that we generate a label directly without referring to a rate. However, we decided not to support that flow, as it would not provide enough insights to you on your label costs. Moreover, the generated label could be costing more than you are willing to pay, and we would have no way of filtering this. Currently, the only carrier known to have this issue is Royal Mail, but please let us know if you're seeing this error message with another carrier!