Buy a shipping label via ShipEngine

Steps

1. Obtain your ShipEngine API key from the ShipEngine dashboard; sandbox keys begin with TEST_ and should be used for all development and testing to avoid purchasing real labels.
2. Add the API-Key header (not Authorization Bearer) to every request: the header name is API-Key and the value is your key; omitting this header or using a different header name returns a 401 Unauthorized.
3. Build a JSON request body specifying shipment details: shipper and recipient address objects, parcel weight and dimensions, and the carrier_id and service_code for the desired carrier service.
4. POST the JSON body to https://api.shipengine.com/v1/labels; a successful response returns a label_id, tracking_number, label_download object with PDF and PNG URLs, and the shipment_cost.
5. Download the label from the label_download.pdf URL and transmit it to the shipping operator or print it; the label is valid immediately upon creation.
6. To void a label (cancel and request a refund), call PUT https://api.shipengine.com/v1/labels/{label_id}/void and check that the approved boolean in the response is true.