Online reporting can be between 15 days and 7 days before expiry according to Immigration website. Mail reporting must be mailed at least 15 days before expiry. So if one tries to do online reporting on day 9, say, and failing, there is not enough time left to do the mailing option! Happened to me (could not get past first page even though I was successful with the previous report) and got admonished on the approval form to send new form next time 15 days before expiry date!

The mailing it 15 days before the report date  is only needed at Bangkok immigration and they will still process if it arrives at least 7 days before the report date.

Other offices just state it needs to be received no later than 7 days before the report date.