Unfortunately Miniinthebox.com is unable to predict these costs ... but their customer service has certainly done their best to communicate with me and try and find a solution.
I think it's best to place several small orders, in order to minimize duty costs. Because of the effort made by miniinthebox's customer service, I have placed and received multiple new orders by now and didn't suffer any extra costs perhaps because I try to stay under the 25 euro amount per order placed.
Update 1: more than a month later now and the LED lights are still working well. Apart from the tax issue, it seems like the quality of the products is OK.
Small remark on the LED lights themselves ... they are somewhat picky on the 12V that is needed. Any deviation makes them flicker but this is a problem that occurs with almost all other brands of LED lights as well. Tip: make sure the 12V adapters in the armature you're using is in good shape.