There should be a simple way to renew your current subscription when you have purchased another hard copy of One Care ( on sale ) by providing an online link where you could just enter your new Product Key to extend another year.
There is. Click Renew in OneCare and sign in to the Billing site that opens; then choose "Prepaid PIN" as a payment. Enter the product key and you've extended your subscription for another 12 months. Simple enough.
Are you signing in to billing with the same LiveID that has a subscription? Try going to http://billing.microsoft.com in your browser and signing in. Choose OneCare then Renew this service, and then enter your key.