How do we find an order from our own system?

Orders can be find by id, by order reference, by email address, and by merchant reference 1. There is also an endpoint for the order's activity history, which is where provider-level responses and errors are recorded. Note that an order consists of several linked entities, so retrieving the order alone does not give you its complete current state. Fetch deliveries, refunds, compensations and releases as well if you need the full picture.