My 90 day reporting is due on the 20th February. On the 26th I am flying to Mandalay and then return a week later. I have a multiple entry visa and Non Imm Retirement Visa. My first question is what leeway is given on the 90 day reporting, would I need to report on or before the 20th since I am going out of the country on the 26th.

The other question is. in Bangkok can we send the 90 day form in by post and what is required, s.a.e, postage etc?

You need to do your report before you leave. Leaving after your report date does not count as doing a report.

You can report 15 day before or 7 days after your report date.

Info for dong reports by mail is here: 90day_notification.pdf

Many thanks for that