Failures, retries and page boundaries
Invalid requests do not debit credits; upstream failures refund the reserved credit. A lost client response is different and may follow a successful charged retrieval. Another successful attempt costs another credit. Your own mapping or storage failure does not establish that the upstream request failed. Include successful empty pages and integration tests in the budget, rather than counting only rows eventually stored in your application.
At most 100 records fit in one request, and a query allows at most 100 pages, requested separately. Those caps do not prove full coverage. HTTP 402 concerns an insufficient balance; 429 concerns rate limiting. /v1/status is public and uncharged but exposes neither account balance nor refund history. Accounts, keys and balances belong to redditdatastream.com rather than a shared network account.