Top up the U.CASH gas wallet that funds gasless x402 relays on Base. Connect MetaMask, see the wallet balance, and add ETH anytime. Sellers opt in to gasless settlement and are billed the gas + 21% per relayed payment; the platform holds the relayer key.
The relayer is one U.CASH platform gas wallet (same address for every seller). Operators: load an agent by api_key to opt it in to gasless billing. Sponsors: look up the wallet by any resource id (no key) and add ETH.
The U.CASH platform key (server-held) funds gasless x402 relays. Top up its Base ETH so relays keep working. One shared wallet for every seller.
Switch MetaMask to a buyer account that holds USDC on Base, fetch the door, and sign. The funded relayer submits on-chain and the resource settles.