Your IP address indicates that you are in Australia. I don't know the cost of OneCare in Australia, but you can create a subscription/activate using a retail product key. In the US, retailers often offer OneCare at a discount, so you would perhaps be saving money by looking for a deal for a retail copy of OneCare.
If you use a retail key, you won't need to provide a credit card. You can also renew with another product key later.
-steve