Hey
I always write 'Airmail' on the front and back of my envelope but if you post it at the post office they will give you a blue 'Airmail' sticker to put on it
Putting the word 'Air Mail' ensures that it will get their faster. I'm not sure if they ship those by boat if it's not written. I know everytime I send to UK, they put a air mail sticker on my envelope.
Technically, the price you paid to post the item should indicate to them that it's airmail (and not surface mail) but it's always good practice You can actually get airmail stickers free at the post office, I'm thinking of getting a roll for my letters
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