Currently using zencart 1.5.7C and paypal express checkout

I have several websites (all use paypal express checkout) and use a single paypal account.

I can't tell the order from which website when I got the paypal payment notification email.

The content in the payment notification email I received from paypal mainly includes:
item name+attribute+item number
I want to add the website name (STORE_NAME), for example, change the item number to STORE_NAME+item number.

So how to achieve this purpose?

Thank you!