@Bobbie ***********
remember when you mail it in use registered mail (so you can track it), mail it 15 days before your actual due date and then don't worry if you don't get the receipt back by the due date. I know people who got the receipt back almost a month after they mailed it in.
Even if you never get the receipt back and your next 90 day report is due, you can print the tracking history showing you mailed it at the correct time and it was signed for by someone at the immigration office and they won't fine you for failure to report.