Hi you can buy a goodybag or credit it's totally upto you. Obviously bags can only be bought with a debit card or credit card.. If you have a voucher just exchange it for a goodybag.... Click the goodybag and tick the box that says pay with airtime..
If your first payment is exactly for the amount of the goodybag you buy you won't have any available credit if you need it. So if you want a £10 bag maybe best to make a top up for £12 or even £15, as the credit remaining after you buy your goodybag is there until you use it at the PAYG rates