Each invoice goes through various stages en route to completion. Here is a breakdown of what those states mean

|State| Sent as | Description |


|Billed| "billed" | A request for payment has been made, but a payment method has not been selected. Therefore, no payment address or spot rate has been assigned, and the expiration timer has not started. 

|Unpaid| "unpaid" | A request for payment has been made, the payment method has been declared, and the expiration timer is ticking. 

|Paid | "paid" | Payment has been received in full, but has not been confirmed on the blockchain 

| Ready to Ship | "ready_to_ship" | The Payment is confirmed on the blockchain and the merchant. Typically, this is when the merchant releases the goods to the customer. 

|Fulfilled| "fulfilled" |An optional state that merchants can use for internal tracking

|Underpaid | "underpaid" | A payment has been received, but does not fulfill the request for payment at the given exchange rate 

|Merchant Review | "merchant_review" | The Expiration window has passed, and the invoice was either underpaid, or a payment was received late

|Refunded | "refunded" | A Merchant requested that a refund be issued for the invoice 

|Invalid | "invalid" |  Any payment on the invoice was flagged as invalid