Detached-signature fallback for x402 wallets that can emit a standalone PAYMENT-SIGNATURE header. THE PRIMARY/RECOMMENDED PATH is for the agent's wallet to pay the quote's `paymentUrl` in-band (e.g. `npx awal@latest x402 pay <paymentUrl>`); use this tool only if your wallet client cannot do that. Charges the agent in USDC on Base mainnet and creates a physical letter for printing and mailing. THIS IS IRREVERSIBLE. Only call after the user has explicitly confirmed the recipient, sender, content, and price returned by create_mail_quote, and after obtaining the signed x402 payment header (see prepare_mail_payment).
Connector