What Agents Buy.

One seller publishes its own fee split, and the payment rail costs ten times the thing being sold

Almost every x402 challenge quotes a single number. api.surplusintelligence.ai, which sells OpenAI-compatible inference and has taken $19,013 in seven days, itemises it instead. Its challenge reports an estimated cost of $0.003302 for a small gpt-4o-mini call, of which $0.003000 is a flat x402 facilitation fee. The inference is about three hundredths of a cent. The rail costs ten times the goods, and the seller says so in the response.

• It then points you somewhere cheaper than itself. The same challenge carries a notice: "For cheapest inference, use SIWE + one-time USDC approval from surplusintelligence.ai website", with a cheapestOption block explaining that the native flow avoids per-request facilitation entirely. A seller telling a buyer that its own advertised payment path is the expensive one is not something this site has seen before, and it is the opposite of the pattern documented everywhere else here.
• The fee is flat, so it decides what can be sold at all. At $0.003 per request the rail is invisible on a dollar purchase and ruinous on a tenth of a cent, which is exactly the band most of this market prices into: the median cheapest offer across every topic in the registry sits between $0.0035 and $0.015. A flat facilitation fee near $0.003 means a large share of listed endpoints cost more to pay for than they cost to buy. That is a structural ceiling on micropayments, not a complaint about one seller.
• It also explains a number that looked strange. This service takes $19,013 from eleven wallets, about $1,728 each, while enrichx402.com takes $406 from sixty. Sellers whose economics only work above the fee attract few, large, committed buyers, and the cheap end attracts many small ones. Both look like demand on a leaderboard sorted by dollars. Only one of them looks like a market.

POST https://api.surplusintelligence.ai/v1/chat/completions
{"model":"gpt-4o-mini","messages":[{"role":"user","content":"ping"}],"max_tokens":5}

from its 402, verbatim:
  "estimatedCostUsd": "0.003302"
  "x402FlatFeeUsd":   "0.003000"
  "notice": "x402 includes a per-request fee to compensate facilitation costs.
             For cheapest inference, use SIWE + one-time USDC approval from
             surplusintelligence.ai website."

it offers two schemes:
  accepts[0]  exact  EIP-3009   pre-charged on max_tokens
  accepts[1]  upto   Permit2    pay only for what you use

the quote moves with the request, which is unusual:
  max_tokens 5      $0.0033      claude-3-opus     $0.0033
  4000-token prompt $0.0036      a model that
  max_tokens 4000   $0.0042      cannot exist      $0.0033