I’ve found that there are two different properties of the order in Magento API available.
order_id and order_increment_id. For sample order they can be something like order_increment_id=100000080 and order_id=81.
The Question Is: What is the difference between them? How they are considered to be used? In the web store UI I see that my order has “Order #” = 100000080. What is order_id property for?
I’ve been working with magento API for almost a year now and can assure you that the only ID you need to use is the order_increment_id. It is used as the main ID in the order.info call. The same is true for the invoice and shipment APIs – they also use the appropriate increment id as the main one.
The order_id, which I believe is the same as entity_id, is the primary key in the sales_order table used to join all the eav tables together. It is used internally in magento, but working with the API you needn’t worry about it.