Whenever shipping parcels overseas, you need to carefully read the terms and conditions of PayPal's seller protection (presuming that you have been paid using PayPal). The main thing is that you must be able to provide proof of postage and track the item online.
Alas here in the UK, airmail does not go above 2Kg, nor does surface mail (unless it is printed papers). Even if you use international signed for, it is only tracked until the point at which it leaves the UK, which is not exactly very helpful.
Parcelforce worldwide does provide carriage for items over 2Kg, and online tracking, but the main issue is the cost - they are very expensive for the service which they provide.
The other option to look at is a courier such as DHL - the website parcel2go.co.uk is very good as it allows you to order parcel delivery through a variety of couriers and thereby gives you some very competitive prices - for example, you can send up to 10Kg to EEC for less than £30. The main downside is that they cannot give you a time when they will collect the parcel, just a day, and even then, we have found that the driver does not always turn up, particularly if it is near a bank holiday - the couriers seem to give preference to their regular business customers, therefore the vans fill up and the poor domestic customers fall by the wayside. One way around this is if you work, to have the parcel collected from your work's address - we often do this and have had no problems with getting stuff picked up (mind you it probably helps that my office is based next to a motorway junction).
We have had a couple of problems of non-delivery, so it is essential that you get a telephone contact for your customer when ordering delivery - this enables the local courier at their end to contact your customer and find out where they live!!
Only other thing with parcel2go is their customer service - emails don't tend to get answered, although a quick telephone call is normally resolved very quickly and if you have something special, they can always arrange an item specific quote for you, which can work out cheaper, particularly when sending to the more far-flung destinations.
Remember - shop around - the post office is not only getting more unreliable, but more expensive !!