Hi guys
It does depend on which country you are sending from, but more importantly how good your local postal/sorting office is

It sounds like Kelly's is pretty good
Your letter will
certainly still get there if you don't write or 'sticker it' to say AIRMAIL, but you do run the risk of it going via
surface mail instead.
The post office
should identify your letter as airmail based on the postage paid, but in practice this often doesn't work. I know this from painful personal experience!!!!
To give you an example, if a letter is clearly marked AIRMAIL and sent to the USA from the UK it would get there in 2-5 days - however, if it was not marked (and the post office were half-asleep!) it could go via surface mail (road/ship/road) and take up to 8 weeks!
I know the surface mail delivery times from the US are about the same.
Don't even get me started on how long it takes to get from the UK to Australia via surface
So whilst I totally agree with KellyMBel that you
shouldn't have to write AIRMAIL (or sticker it) I personally wouldn't take the chance. After all it doesn't take a second to make sure!
Just my penny worth . . . . .
Ian
