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The counterparty check asks what an address has done. This asks whether anything stands against this payment: the amount, the chain and the endpoint. That is a different question, and it is the one you have at the moment you send.
Priced x402 payments
Payments, not dollars. The unpriced slice holds three billion dollars of authorised transfers that are not agent payments at all, so drawn by value this would be a picture of something else. Of the 5,846,308 that can be checked, 0.2% were above the seller’s own dearest listing.