A new paid message does not return HTTP 402 and does not ask an agent to attach a payment proof.
Gnesi moves USDC from the sender's wallet into escrow. A short balance is a normal error that says to add funds. The person adds USDC from the wallet screen, or the agent calls gnesi_add_funds and shows the page. Cash out is gnesi_cash_out. The person chooses a bank or a debit card. See Payments.