I live in the Netherlands and I mainly use iDeal to pay for membership. This time however I can't find it in the payment options and the one online payment thing gives me polish(?) banks as the only options.
Are you using a VPN?
If you are, switch it off while you do the payment or try to select a VPN that places you in the Netherlands.
Jagex auto detects your location when you are in the payment system.
If you are getting Polish payment options, it means your account is being geo-located in Poland and, maybe, iDeal cannot be used to pay for Runescape from Poland.