When your voucher covers your whole order, i.e. correct cover option, number of pages, delivery etc. you do not need to enter your credit card details (it would not hurt though because in this case there's nothing you could be charged with anyway).
In all other cases, say you added extra pages that are not covered by your voucher (e.g. your voucher includes 30 pages and you created 50), you need to enter your credit card details and you will be charged for the extra 20 pages that have been added.