Paying import and duty at airport?

Has anyone had any experience of this at Zaventem airport?

I'm just curious about how it would work and at what point would you have to pay the fees?
I'm guessing you would have to declare the item when going through passport control and then they would want to inspect the item but is that where you give them the money or are you billed for it later.

Another point is where can I find the actual VAT rate for something like optics or more precisely a telescope. One online duty calculator mentions that this would fall under a reduced VAT rate of 6% but I have no idea where they pulled that from.
Is there a rate for scientific equipment as I reckon that's a fair label on a telescope.

Frankly I don't trust Belgian customs to have any idea of it themselves but this may be a better way to save a long drawn out back and forth if sent by post.