if shopping vouchers are more motivation to quit than your baby's health maybe you need to have a re-evaluation of your priorities.
Very snappy sentiment but you and the person who voiced this are ignoring the elephant in the room... Do you not realise that there are still people who don't accept that smoking in pregnancy is an issue?!
The world and her sister all know someone who smoked through out 3+ pregnancies and the baby was "fine". For these women, as far as they're concerned, smoking is just a tiny increased risk that won't actually affect them. Giving up cigarettes, which is not pleasant, for, as they see it, no concrete benefit, is not going to happen. Your 'reward', so to speak, for giving up for the sake of the baby, is sitting in a delivery ward thinking, "if I hadn't given up, maybe this baby would be less healthy". That's not receiving a reward, that's just avoiding a penalty!
As it happens, some women deliberately continue smoking in order to keep the baby small, to avoid tears.
Shopping vouchers, however, are tangible positive rewards; you can see them and you know you will definitely get them for the stuff you need for the baby if you can get through this week before your appointment with the midwife without a cigarette.