What stamp do you have to buy for ARC stamped addressed envelope?

What stamp do you need to put on your stamped addressed envelope that the NIA send you your new ARC in?

Last time I put a regular stamp on it, but the lady told me I need some other stamp. Maybe a priority one or does it even have to be registered mail?

Last time I just paid them and they took care ofit, they already have the stamped envelope ready.

use 掛號 as that is traceable through Chung Hwa Post website

Just 掛號, or 雙掛號? :ponder:

or 掛號限時