Thailand Postal Service
Sending letters and postcards is no problem. Well, it is true that sometimes there are long, slow lines at the post office. Aside from that, no problem. Occasionally, people will complain that letters go missing. While this is no doubt true in some instances, yours truly has only experienced missing mail one time in well over a decade of living in the Land of Smiles.
Sending packages is easy too. You can buy several different sizes of boxes right at the post office. String and tape comes with the box, but they're a little stingy with the tape so you might want to bring your own.
Example Postal Rates (Economy Air, Thailand to USA) - 1kg = 2.2 lbs
Usually EMS letters or packages get to the USA or UK in less than a week. Sea Mail takes one to three months. Packages sent via Economy Air, as in the example above, usually take one to two weeks.
While some countries require some form of ID to send letters or packages - especially of the registered variety - Thailand does not.
Mailing a registered letter or package does require filling out a form which includes your name and address. You will then receive a receipt which you keep until you are sure that your mail has arrived.
It's as simple as that.