I have read that the Article Processing Charge (APC) for publication in this open access Special Issue of Materials is 1800 CHF. I am wondering wether the Journal applies the same APC for all the papers by the same author.
It depends on the policy of the journal on article processing charges. I suggest you write a letter to editor and request for waiver of APC. He will reply and you can then bargain. I hope this helps.
I am sorry but only noted your question today: I am happy to offer a 50% discount on your next paper in special issue of Materials. I hope this could be useful.