I filed the request away as a suggested improvement to AR and we'll see whether anyone else has had a similar request. It hasn't been asked for before to my knowledge.
Personally, I would send the calculation rules to the requester and then have a conversation about paying the invoices in a more timely manner. With the cost of money so low these days, they are not earning anything hanging onto their cash. If they insisted on you calculating a breakdown, I'd offer to do so and charge for the service (since your time isn't free). I can hardly imagine a scenario where the interest charges are material enough to require allocation back to the original PO.
And surely PO's aren't raised on the expectation that there will be interest charges for late payment!?!
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Andrew Bates