All income won't be taxed. It's dependent on each individual circumstance. A person might be a tax resident in another country which would take precedence or might only transfer pension money which may be non-taxable in Thailand. Too many variables
It's a five minute stroll to my closest PO, postage and return with EMS about 70 baht, two or three photocopies a few baht. Box ticked. I laugh when I read about people spending half a day at immigration just for a check-in..