PayPal customization question

I’m trying to change the subject line in our PayPal notification e-mails. Currently they look like this:

How can I change that “Payment Accepted!” line? We’ve gotten complaints from buyers that they thought it was spam because of it. I’d like to put in something more professional, but PayPal’s documentation is for so convoluted and crappy, I can’t find out if it’s possible.

Anyone?