You might try listing it as a "Room xxx" instead. The issue is typically that the non-USPS carriers cannot make deliveries to PO Boxes (on behalf of Apple or anyone else). A lot of times, University offices have a secondary address format that uses a street address instead of a PO Box for this reason. Usually there are room numbers associated with them; it might be that the "Box" just flagged a warning. As long as there's a person at a desk / office in your dorm (like there usually are) who accepts the mail on your behalf and puts it in the mailbox, there shouldn't be an issue using FedEx / UPS / etc.