Airmail - definition of letter and packet please!

[Bunnygirl]

Good morning, new member here!
Please can someone help me with this rather grey area regarding airmail post.
What constitutes an airmail letter and an airmail packet?
I don't believe that Royal Mail actually gives a sizing guide anywhere like they do for UK post.

With UK post, a small item for example a make up item under 100g say sent in a jiffy bag will post in UK as large 'letter' rate.

My question is can I post same make up item by airmail as a letter or does it need to go as a packet?

Its very confusing trying to find out the difference between RM letter and packet rates for airmail.
Please can someone clarify this for me. I don't want my items delayed or returned to me for posting incorrectly but likewise I do not want to pay more by using the wrong service.
Thank you muchly!

[capitalbiker]

all you should need is here

http://www.royalmail.com/delivery/deliv ... al/airmail

there's an ask an assitant bit at the foot of some of the pages

anything can be sent as a letter up to 2kg provided you are prepared to pay the price