Breaking change for API users.
When your account has 2 or more active carriers of the same type and you create a shipment without setup.carrier_id, the request now returns HTTP 400 (MULTIPLE_ACTIVE_CARRIERS) instead of a label. Previously, one account was selected arbitrarily.
Add setup.carrier_id to choose which account to charge.
Accounts with a single active carrier of that type are unaffected.
See the API reference (developer.shipandco.com).
