Mark the payment intent as confirmed, and move the order to the complete state

const options = {method: 'PATCH', headers: {'X-Spree-Order-Token': '<api-key>'}};

fetch('https://demo.spreecommerce.org/api/v2/storefront/stripe/payment_intents/{id}/confirm', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

{
  "data": {
    "id": "980a80d5-f51b-4065-b3a0-622fc5cb6bf6",
    "type": "payment_intent",
    "attributes": {
      "stripe_id": "pi_3QhUuj2ESifGlJez0ADSS4tu",
      "client_secret": "pi_3KQRG2DcAgmy0ota09WCW6nf_secret_8XTxRbOyg7oEkvvIOvnP75dRw",
      "ephemeral_key_secret": "ek_test_YWNjdF8xTGtSb1lJaFIwZ0llZ0llyG1LbFhsVU9XWHY1SlNyMXRKVzNJb0hyQ2JZam1HY28_007lGgvGaA",
      "customer_id": "cus_Q9uqUz6gVQFku2",
      "amount": "44.0",
      "stripe_payment_method_id": "pm_1Qedd42ESifGlJez0IOx5AZ3"
    }
  }
}