At the time writing (mid-June 2012) the rate we're legally bound to charge for courier services is 20%. However, as this rate has changed thrice in recent years (17.5% to 15% to 17.5% and then to the current 20%) it is possible the rate will change in the future.
If the VAT rate is important to you, the HMRC publish theheadline rates, and you can check the prices there.
Aren't Postal charges exempt from VAT?
The current rules state that Postal services provided by the Post Office are exempt from VAT, but this exemption does not extend to similar services provided by other suppliers, even where this might be seen as being in direct competition with the Post Office.'
[Source: HMRC - retrieved 15/06/12]
But what about on non-EU-Deliveries?
You will notice, however, that parcel deliveries to non-EU locations do not attract VAT.
Will I be told when VAT is being charged?
When you conduct a quick quote, the page that lists the available services will detail which will incur VAT.