Letting an agent pay for and renew its own API access with x402 and a budgeted wallet

I built this, so treat it as a writeup of our own implementation. The flow is: GET /api/shop/agent returns an x402 offer for $29 and 30 days, a CoinPay wallet registered with per-payment, daily, and total limits pays it, and the Power Key comes back in the same response for the agent to store. Before expiry the agent can schedule the next payment and swap the key itself. The parts I think are worth arguing about are the guardrails: budget is checked before each settlement, we never hold the agent wallet signer, and if funds or budget run out the payment is refused and the old key just runs to its expiration instead of anything being extended. Workflow, client example and gateway source are linked in the post.