I posted before that I will be away from my usual province at the time my report is due.

It's now pretty certain that I will be going to Cambodia for a week or so a few days days after the ninety day reporting date. Will this be OK, as I know there is a seven day grace period after the actual date if you report at the office? Or will I still need to do a postal report as well?

If you are still in Thailand on 90 days you must report. You can do this by mail or up to 7 days (15 in some places) earlier in person.

If you have left Thailand before 90 days there is no need to report.

The clock starts again on your return (count your entry day as day 1)

You wil be staying in Thaliand for 90 days, so you must report.