Can I simply hand in the paperwork required for a postal application,together with the stamped return envelope to the immigration desk.

Since the delays for 90 day reports can be many hours,it would be much easier for them to treat it as a postal application rather than add another waiting body.

That is not one of the options.

The foreigner makes the notification in person, or

The foreigner authorises another person to make the notification, or

The foreigner makes the notification by registered mail.

Registered mail only costs about 20 baht and you have proof you send the 90 day report.