Are you unsure which product category your item falls into and therefore which commission belongs to it? By uploading the item in your seller account you can check both the product category and the corresponding commission.
You sell an item for €100 including VAT. From that, you first calculate the commission percentage – say 5%. So that’s €5. Then you add the fixed commission amount, i.e. €5 + €0.83. So for this item with a sales price of €100, you pay a commission of €5.83 excluding VAT.
Are you and your company based in the Netherlands? Then you will pay the Dutch VAT rate of 21% on the commission and this will also be charged on your Sales Invoice. Do you sell from Belgium and do you have a valid Belgian VAT identification number? Then the commissions are in principle calculated exclusive of VAT and are therefore also exclusive of Dutch VAT on your Sales Invoice.