如何获得业务电子邮件ID以响应paypal api

I Integrated Paypal in my website, I give amount and business email along with cmd to paypal when payment is successful it get back to my return URL with a query string

amt=67.00&cc=USD&st=Completed&tx=XXXXXXXXXXXXXXX

But, I also want to get Email(Business) in my query string in which the payment has been done because I want to save it in my database.

What I want is some what like this:

amt=67.00&cc=USD&st=Completed&tx=XXXXXXXXXXXXXXX&email=example@paypal.com

When you doing integration, at that time you have already merchant details like email, so you can directly put to database, why you want to get in return URL?