Before you can send billing documents to an account by email, you must designate a Salesforce Contact with a valid email address as the billing contact for that account.
To create a billing contact for an account:
If a contact does not exist for the account, click New Contact on the Contacts related list of the Account you are creating the contact for, then complete the Contact fields as described in the Salesforce Help.
Enter a valid email address for the contact.
Select the Billing Contact checkbox.
Click Save.
Repeat these steps for each account that you want to email billing documents to.