Pure luck. The law says all postal shipments under 150kn(shipping you payed for it included) does not go under custom inspection, but in practice everything under 300kn can easily pass most of the time. It also helps if the person you buy from(eBay mainly) declares the item with a lower value so that it goes by customs unnoticed. But it's luck most of all, with thousands of packages that pass every day, they can't inspect all of them.