UIViewController.swift

func paymentAuthorizationViewController(_ controller: PKPaymentAuthorizationViewController, didAuthorizePayment payment: PKPayment, handler completion: @escaping (PKPaymentAuthorizationResult) -> Void) {
        // Use Stripe to charge the user
        STPAPIClient.shared().createToken(with: payment) { (stripeToken, error) in
            guard error == nil, let stripeToken = stripeToken else {
                print(error!)
                return
            }

            let url = URL(string:"http://localhost:3000/pay")
            guard let apiUrl = url else {
                print("Error creating url")
                return
            }

            var request = URLRequest(url: apiUrl)
            request.httpMethod = "POST"
            request.setValue("application/json", forHTTPHeaderField: "Content-Type")
            request.setValue("application/json", forHTTPHeaderField: "Accept")

            // Stripe doesn't allow decimal, so have to convert the amount
            // into cents by multiplying by 100
            let body:[String : Any] = ["stripeToken" : stripeToken.tokenId,
                                       "amount" : 300,
                                       "description" : "Purchase of a t-shirt"]

            // Convert the body to JSON
            try? request.httpBody = JSONSerialization.data(withJSONObject: body, options: .prettyPrinted)

            // Make an HTTP request to our backend
            let task = URLSession.shared.dataTask(with: request) {data, response, error in
                guard error == nil else {
                    print (error!)
                    completion(PKPaymentAuthorizationResult(status: .failure, errors: nil))
                    return
                }

                guard let response = response else {
                    print ("Empty or erronous response")
                    completion(PKPaymentAuthorizationResult(status: .failure, errors: nil))
                    return
                }
                
                print (response)
                
                if let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode == 200 {
                    // Once the payment is successful, show the user that the purchase has been successful
                    completion(PKPaymentAuthorizationResult(status: .success, errors: nil))
                } else {
                    completion(PKPaymentAuthorizationResult(status: .failure, errors: nil))
                }
            }

            task.resume()
        }
        
        completion(PKPaymentAuthorizationResult(status: .success, errors: nil))
    }