curl -X POST "https://api.adrelay.dev/v1/billing/create-checkout-session" \
  -H "X-API-Key: $ADRELAY_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"plan": "pro"}'

const response = await fetch(
  "https://api.adrelay.dev/v1/billing/create-checkout-session",
  {
    method: "POST",
    headers: {
      "X-API-Key": process.env.ADRELAY_API_KEY,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({ plan: "pro" }),
  }
);

const data = await response.json();
// Redirect user to data.checkout_url

import requests

response = requests.post(
    "https://api.adrelay.dev/v1/billing/create-checkout-session",
    headers={
        "X-API-Key": "your-api-key-here",
        "Content-Type": "application/json",
    },
    json={"plan": "pro"},
)

data = response.json()
# Redirect user to data["checkout_url"]

{
  "checkout_url": "https://checkout.stripe.com/c/pay/cs_live_...",
  "session_id": "cs_live_abc123"
}

{
  "error": "Invalid plan. Must be one of: starter, pro, enterprise.",
  "code": "VALIDATION_ERROR",
  "timestamp": 1711036800,
  "request_id": "req_abc123"
}