A worked example
A buying agent parses 10,000 invoices in a month at €0.02. The buyer pays €200 in listed price plus €6 commission, so €206. The seller receives €194. If 120 of those calls return a 5xx and are retried successfully once, only 10,000 calls are charged. If the buyer sends malformed payloads and gets 4xx, those are charged, because the work was done.