Pick up the phone. Call the head of admissions. Ask for clarification about the letter. Be nice when you talk with them. Focus on the need for clarification, and your concern about the difference between the date on the letter and the postmark date. Perhaps there has been an error, and your kid is still in. Perhaps your kid really isn’t in. And if your kid is not in, ask admissions to intervene for you with billing so that you get the deposit back.