We certainly understand the need for this requirement and we are still committed on providing custom fields on invoices.
We recently released PDF invoices as attachments for the "Invoice Receipt" email notification. Now, the "Invoice Receipt" email template includes text content along with mail merge field support. So you can add custom fields to the email content that will be sent along with the invoice receipt attachment.
The only thing missing here is that custom fields are still only supported at the customer object level and not the subscription level. We will keep you updated as we make progress on this.
We need to be able to add custom field data to the invoices that are issued to customers.
We use the system to charge for internet services and one customer might have several subscriptions for the same plan, but for a specific domain name or ADSL phone line. We have this information stored in the system in custom fields but we are unable to add it to the invoice that is sent to the customer, so the customer does now know what they are being invoiced for.
I understand that this is planned but thought I would add it here as it is critical for us.