Be specific as to:
1) which order you're returning,
2) why you were dissatisfied, and
3) what you expect them to do about it. Ask for confirmation on when the situation will be remedied.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I am returning a shipment of <contents and order number>. We cannot accept this shipment because <reason for refusal>.
Please <required correction>.
Please advise as to when we might expect <corrective action>.
encl: <List of enclosed items goes here>