To use products in orders, create products with the UI, and then list and get products with the API.
For full details of how to create products in the UI, see Create your product catalogue.
The system product id is the unique system identifier for the product.
In the UI, when you create a product, you can create a 10-digit merchant product_id, and if you do not create one or if you delete the existing one, BillPro creates one automatically. You can modify the merchant product_id in the UI. Ideally, the product_id should be unique.
You can filter the list of products by the merchant product_id.
You must use the order endpoint to match the product's product_type. In orders in the UI, you can switch between single payment and instalment order types with a split payments option, but in the API, this option is not available.

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Product ID when creating an order
When you add a product to an order, you must specify the product using the system product id. In an order body object, this attribute is called the product_id!