If you set up a PayPal account, you should be able to buy a Bond directly from the website.
If you buy a RuneScape Pre-Paid Giftcard, it works a bit differently. RuneScape Pre-Paid Giftcards can be bought online and in shops. The pre-paid giftcards have set packages - there are two cards, a lower value one and a higher value one. Instructions about how to redeem: How to redeem a Prepaid Card
NOTE: you will get some membership when redeeming a RS pre-paid giftcard for Bonds. The lower value card gives you one Bond and 7 days membership. The higher value card gives you three Bonds and 7 days membership.
If you truly do not want membership, don't buy the giftcard, use PayPal instead.