It will be cool to include credit into invoice mail, because I don't want to visit my panel if all works fine and I've pay for some months advance.
It would be great if the invoice could include a comment like "You currently have $123.45 in credit which will be used to pay this invoice" or similar. For instance it might say "You currently have $1.23 in paypal credit which is not sufficient to pay this invoice". For accounts with credit cards attached the comment could read "Your credit card will be billed on date YYYY/MM/DD".
Interesting suggestion, something we can look into if we get more up votes.
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