in denmark they sort goods in two ways, things from within the eu (which does not go to the customs). The rest is opened by customs, who first looks for something that declares the value, if they can find that, they just do the math for the tax+costums and hand it over to the post service (we only have one). The post service will not give it to you before you have paid.

if they can't find something that tells the value, they keep it and send a letter where they demand some proof of value before they pass it on...