A payment gateway takes significantly more code than acting on a single trigger. CBSubs needs the code in place to handle validation, payment, status changes, refunding, cancelling, and much more. So no acting on a single trigger will not accomplish this. I would need to study the Authorize.net hosted payment page API to better suggest what gateway would be a good starting point, but as a general suggestion you may want to look at GoogleCheckout for example as I took extra care to write and format it cleanly so it's easier to understand. Ogone is also another great example that uses hosted payment page usage.