As per the thread below, Ive recently come across this with several ebay items (from US sellers).

Has anyone found out why they are charging extra on top of the 15% for GST?

https://www.geekzone.co.nz/forums.asp?topicid=277390&forumid=48 

The import charges for this should be around $11 AFAIK.

Yes I realise its only $5 in this case but it seems very odd to be charging beyond the 15% for GST.

And heres another one, same thing, extra $5? Doesnt seem value related as this purchase was less in value and shipping.