They contain information about the customer who was billed, the items that were included, and the transactions related to the invoice.
Pay close attention to the construction of this object.
Invoices must contain an array of one or more line items that were billed. Line item objects represent the kind of product or service being billed, and relevant details about them.
Invoices usually always contain an array of one or more transactions. Transaction objects represent payment or refund attempts related to the invoice.