discount/gift certificates redeemed at shopping cart page

I've just purchase some goods from a merchant that uses opencart and there was a radio button for dscount coupons and gift vouchers on th shopping cart page so it should be doable. Just needs someone with more brains than me.:D the reason I ask is I must get 1 or2 calls a day from customers asking how they get there discount and yes there is a note on the shopping cart page checkout page and I've put a notice with my logo on the paypal page, so I thought if they could claim there discount at the shopping cart page it'd save my poor assistant having to field these types of calls.
thinking caps on guys :smartalec: