The pay per return fee is between you and Intuit.  Why would you send it to your client to back up whatever fee you have agreed with that client?

In any case, you would have already paid for the credits you PPR returns upfront and Intuit would just deduct a credit for each return you prepare.  The invoice was issued at the time you made the purchase and should have been sent to you at the email address you use to sign up for Intuit.