1. A method for application-based billing, the method comprising: generating an application-based billing request for purchase approval within a client device making the purchase, the client device being a wireless subscriber unit configured to communicate with a wireless subscriber billing system; transmitting the billing request including a subscriber identification (SID) to the billing system; generating a validation response to the billing request at the billing system; and transmitting the validation response to the client device, wherein the billing request is an in-app billing request that requests activation of one or more additional features for an existing application configured for execution on the client device, and/or wherein the validation response is based on a roaming status of the client device.