Auto responders - thoughts on most efficient way to bill clients?

Great to see auto responders (ARs) launched.  Just a few questions I need to think through on efficient billing clients. (I note the charge is 1c per email).

Current status:
With email campaigns it's fairly efficient for me at the moment. I purchase a bunch of credits every 2 months.
When I get the send notification for each client campaign I raise an invoice to them to cover the send fee + my management fee for newsletter production.

Future Status:
(i) Email campaigns - I continue to work as above.
(ii) ARs though?...
Lets say I'm running 10 auto responder campaigns across 10 different clients.
Let's say the average sends per client is 200 emails per month ($2).

Question:
I need an efficient way of tracking how many AR emails have been sent for each client so I can bill them as part of their existing monthly invoice?
What will be the best way for me to do this without poring over lots of different reports and creating admin for myself?
Nb. Ideally I do not want the client to have to buy their own credits. My job is to make life easy for them!

Thoughts?

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