I also posted the question to Paypal X.com and I got an answer as follows. I start working on it now. Thanks!
In the code that your cart script sends to PayPal, it is the value you pass
for 'business' that determines where the payment should go to.
So you can have a script on your end that will dynamically change this value
as shown in the example below before posting to PayPal.
<input type="hidden" name="business" value="firstname.lastname@example.org
You can find a list of the standard variables here
I found "business" in confirm.php page. Postcode is variable and when the particular value is placed, it triggers action to go to one of confirm.php pages.Is that right? So postcode gets inserted in checkout.php which provides billing information. So where do I put PHP code to allocate a customer to particular confirm.php page? I am only a designer and not really good at coding. Can anybody help this?