This endpoint generates a draft invoice for a specified customer. An invoice remains in draft status until it is finalized.

1. You can specify the id of an existing customer in "customer_id" field or you can add a new customer by providing customer's name and email in "customer" object.
2. You can then provide one/multiple invoice line items having type,
   1. As “line_item” i.e., using the existing price created in Speed. Click here to know more about how to create prices in Speed.
   2. As “custom_line_item” i.e., by providing new price details runtime (i.e. item name and unit amount).
   3. You can also provide line_item and custom_line_item together.
3. Define your preferred base currency (fiat or SATS/BTC) to create an invoice. A three-lettered ISO-compliant currency name must be used.