When shipping international orders with a value of more than $2500, an ITN number is required.
You can apply for an ITN Number at the link below.
http://simplifiedtradesolutions.com/get-an-aes-itn/
Once you obtain the ITN number, you will need to provide it as part of the Customs Form information when shipping the international order - see customs form screenshot below.
Also if you need information on why you need an ITN number, you can find more information at the link below.