you sent in too late

With USPS, since you mailed it on 9/29, it did arrive on 9/30 either noon or by 3pm. However, the service center mailbox was picked up each day in the moring around 5am to 6am. So, you missed the 9/30 pick up. It actually got it on 10/1. It may have relaxed the rule in some cases. But obviously, you had bad luck.