Although billing errors are frustrating, it doesn't help the business relationship to adopt an angry tone. It is, however, imperative to get the dispute in writing, as soon as possible.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
Unfortunately, we are unable to process <reference invoice> because <reason>.
<Proposed remedy.> We will be happy to remit payment upon receipt of a correct invoice. Thank you for your attention to this matter.
encl: <List of enclosed items goes here>