Can we call them just "zegels", in the same way in English we usually just say "stamps" rather than "postage stamps"?
Also don't forget to mention all kind of zegels linked to zegeltjes plakken which is (or used to be?) almost a national sport. For instance at supermarkets you get zegeltjes for every x amount of money spent. Then you have to stick these zegeltjes on a spaarkaart and if you hand in a full spaarkaart you get e.g. a discount or some products for free.