curl --request POST \
  --url https://sandbox.api.withgale.com/v1/v1/checkout \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "customer": {
    "first_name": "<string>",
    "last_name": "<string>",
    "email": "<string>"
  },
  "line_items": [
    {
      "product_id": "<string>",
      "price_cents": 123,
      "quantity": 123
    }
  ],
  "shipping_amount_cents": 123,
  "tax_amount_cents": 123,
  "discount_amount_cents": 123,
  "total_amount_cents": 123,
  "currency": "<string>",
  "external_cart_id": "<string>"
}'

{
  "checkout_id": "<string>",
  "status": "UNPAID",
  "checkout_url": "<string>",
  "expires_at": "2023-11-07T05:31:56Z"
}