Step 2 for a buyer: returns the live payment requirements for funding a packet — the onchain door's own x402 challenge (accepts[], exact amount, asset, chain, pay-to address) read from that door at this moment, and, when a card rail is mounted on this host, its hosted checkout URL. To chip in a slice instead of the whole wage, include contributeUsd in the funding POST the challenge names; omit it to pay everything the packet still needs. You pay from YOUR own wallet or card; this server holds no funds and no keys, takes no payment, and marks nothing funded. A packet becomes funded only when a payment actually settles and is verified. Not ready to pay, or want to signal backing without money moving yet? prepare_backing is the FREE sibling door — a signed pledge, no payment required.