See proposal from Vicki, http://lists.w3.org/Archives/Public/public-vocabs/2014Dec/0031.html

"Schema.org currently has http://schema.org/Offer for offering items for
sale/lease. It has http://schema.org/Order to model when a purchase has been made,
but there is no type for marking up Invoices requesting payment.

To that end, we are proposing adding a new type: schema.org/Invoice where
an invoice is defined as "a statement of the money due for goods or
services; a bill.""