Adding Alertpay Payment Gateway

We have a website already built. The site is built using PHP. There is a credit style system already installed using Paypal however, we no longer want to use Paypal and want to convert to Alertpay instead. The job is to install Alertpay throughout the website, the user will be able to "buy" credits through this gateway and then automatically returned to their dashboard. Once they have used their credits they will be able to purchase more... We also want the website to be based on offering a "7 Day Trial" to the user. So initially they will have to enter all valid credit card info to be able to sign up to the website. If they do not cancel within the first 7 days they automatically are billed for their first set of credits on a monthly basis.. Any questions please ask.