I had updated the email address in account settings prior to sending the campaign to ensure that the tax invoice would be sent to the correct email address. But it was still sent to the old email, which doesn't exist anymore (hasn't existed for over a year). Because we are paying using AUD, we can't go into the billing history to obtain another copy of the invoice. I need to request another invoice to provide to the accounts department.
Is there another place that I need to update the email details so that future invoices are sent to the correct email?
The invoice should always go to the address in the account settings for the person paying (i.e the client account owner in the case of the client paying).
You can get all invoices from inside your account though, no matter who is paying. If the client is paying, you just need to go to "Client Settings" and then "Client billing history" to access them. Alternatively, login as the client and just go to "Billing".