Paypal IPN using multiple webpages

I have a website with 3 subdomains (am probably going to add a fourth soon). Each operates with a database using php and requires paypal's instant payment notification. Right now just the main site is working because I am just able to tell paypal to notify the main site (under the profile section in paypal). I need to have paypal notify the main site AND all my subdomains using the same paypal email account . I know this can be done because I read the IPN paypal manual. It said you had to change the html (or in my site's case it may be php scripting) and it would override the profile web address. I need someone who has experience doing this and can make my sites work correctly. My sites all feature instant user posting and right now without the IPN, the sites aren't recognizing the purchases being made and allowing the users to post. I will private message the webaddresses to anyone requiring a closer look before they decide to take this on. I am placing the budget at the small end range because I really don't know what this involves. If this is not enough, please post what you think it will cost. Thanks.