Website Payments Standard and IPN integration with ecart and Dreamweaver

I am new to implementing payment options on a website. I am using Dreamweaver and have purchased ecart. I would like someone to explain the steps needed to implement Paypal's Instant Payment Notification (IPN) with ecart. I have done some experimenting and I have managed to set everything up to where the database is being updated successfully from the checkout page and I get to the paypal site where you log in to make the payment. ecart seems to create the other pages correctly also eg the confirm and success etc. It looks like the payment process would work fine if I continued from here but before I take the trial run any further I would like to know how I can achieve to only update the database if the IPN returns with a successful payment transaction completed. I have very little PHP knowledge but having said that I am a quick learner. I would appreciate any advice on how to get things up and running for now, as the saying goes, I can add bells and whistles later when I have gained more experience with PHP. I am looking to implement a few add to cart buttons, one for Annual Subscription, There will be 4 other add to cart buttons for other products on which I would like to setup the shipping charge to depend on the Country in the address field of the checkout form. Any information to direct me to achieving the goal or links to other info sources which may help would be greatly appreciated.