shipping_method_id, which you can retrieve from the Retrieve a list of shipping methods endpoint.
If you don’t have a direct carrier contract connected to your Sendcloud account, the rates returned via this endpoint will be provided by Sendcloud, discounted based on your current subscription plan. If you have connected your own carrier contract then rates for those carriers will be null unless you have uploaded your own contract pricing.
Carriers that use pricing based on shipping zones (such as Spanish carriers which charge different rates based on the postal code of the shipment) are not supported.