Is it like a bill or something ?
I was in a comic book shop and the owner was worried because he couldn't find one.
They must be pretty important things.
Yes, of sorts. Invoices can be for buyers or sellers, usually showing amounts paid or to be paid for a service or products. In terms of international sales, they show money paid, quantity of items included in an order and shipping costs. I work for the jobcentre and we recieve them from royal mail for sending letters and packets.
http://en.wikipedia.org/wiki/Invoice
"Invoice" referes to the volume of speech mothers insist kids use whilst indoors.
"Stop shouting! Please use your invoice when you're in the den playing Clue, children!"
:teddyr: