We make box springs that we sell at our own store. We procure the mattresses for those box springs from a Dutch mattress manufacturer. Do I have to pay a waste management contribution?

Seeing as all manufacturers are under an obligation to pay the waste management contribution to the foundation, your supplier will pay the contribution (if all is well). Your supplier will make this known to you by stating it on the receipt, for example. The contribution is charged only on the mattresses of a box spring combination, i.e. not on the box spring. For a box spring with 2 mattresses, however, a waste management contribution will be charged for each mattress.