There is a set minimum for these duties. Under that minimum customs simply doesn't bother. In the Netherlands there's actually two minima. Under 22 euro you pay nothing, above 22 euro you only pay VAT (at 6% or 21%, depending on the product) and above 150 euro you pay VAT plus import duty. Import duty varies heavily by product and the import duty % is mainly a political decision - to a large extent influenced by the desire to protect our own internal market.
Don't expect that you just have to pay the VAT and import duty though. The post office/shipper will want to have some money off you as well for the handling/invoicing of the VAT/import duty. In NL, this is around 7.5 to 12.5 euros from memory.
Oh, and everything is based on the "declared value" as listed by the shipper.
On the bright side, if Aircraft Spruce did things right, they have invoiced you tax-free.