I thought that the whole idea of being able to report by post to the immigration department in Bangkok was

to help people who live very far from thier respective immigration offices and especially older retirees.

I am now confused by posts implying only those who live near Bangkok can apply there.

You could always mail your 90 day report to your local immigration office. Only a few offices refuse it, but the law itself even state that you can mail your report.

Now you just have to make use of your local immigration office and cannot chose any more.

Check with your local office if you can do it by mail or not.