This kept happening to me. Then I was told by someone who works at the post office, it's because it's insured. If they see that, it's looked at much more closely. The problem is if you do not get it insured and it gets lost or damaged your well out of pocket. If you do get insurance, your looking at a bigger bill, with the import duty, V.A.T. and their fixed charges!!, and then the delays!
So to sum up the postal services are shite, and we're stuck with them.