Below is my code which I largely put together based on the Quickstart Guide (https://docs.stripe.com/payments/quickstart) plus the Address Element section of Stripe documentation (https://docs.stripe.com/elements/address-element/collect-addresses).

import UIKit
import StripePaymentSheet

class CheckoutViewController: UIViewController {

    private static let backendURL = URL(string: "https://sundealit.com/stripetest")!

    private var paymentIntentClientSecret: String?
    private var addressViewController: AddressViewController?
    private var addressDetails: AddressViewController.AddressDetails?

    private lazy var payButton: UIButton = {
        let button = UIButton(type: .custom)
        button.setTitle("Pay now", for: .normal)
        button.backgroundColor = .systemIndigo
        button.layer.cornerRadius = 5
        button.contentEdgeInsets = UIEdgeInsets(top: 12, left: 12, bottom: 12, right: 12)
        button.addTarget(self, action: #selector(pay), for: .touchUpInside)
        button.translatesAutoresizingMaskIntoConstraints = false
        button.isEnabled = false // Initially disabled
        return button
    }()
    
    private lazy var addressButton: UIButton = {
        let button = UIButton(type: .custom)
        button.setTitle("Add Address", for: .normal)
        button.backgroundColor = .systemIndigo
        button.layer.cornerRadius = 5
        button.contentEdgeInsets = UIEdgeInsets(top: 12, left: 12, bottom: 12, right: 12)
        button.addTarget(self, action: #selector(address), for: .touchUpInside)
        button.translatesAutoresizingMaskIntoConstraints = false
        button.isEnabled = true // Enable this button initially so user can add address
        return button
    }()
    
    private var addressConfiguration: AddressViewController.Configuration {
        return AddressViewController.Configuration(
          additionalFields: .init(phone: .required),
          allowedCountries: ["US", "CA", "GB", "HK"],
          title: "Shipping Address"
        )
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemBackground
        view.addSubview(payButton)
        view.addSubview(addressButton)

        NSLayoutConstraint.activate([
            payButton.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 16),
            payButton.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: -16),
            payButton.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: -16)
        ])
        
        NSLayoutConstraint.activate([
            addressButton.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 16),
            addressButton.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: -16),
            addressButton.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 16)
        ])

        self.fetchPaymentIntent()
    }

    func fetchPaymentIntent() {
        let url = Self.backendURL.appendingPathComponent("/create-payment-intent")

        let shoppingCartContent: [String: Any] = [
            "items": [
                ["id": "xl-shirt"]
            ]
        ]

        var request = URLRequest(url: url)
        request.httpMethod = "POST"
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.httpBody = try? JSONSerialization.data(withJSONObject: shoppingCartContent)

        let task = URLSession.shared.dataTask(with: request, completionHandler: { [weak self] (data, response, error) in
            guard
                let response = response as? HTTPURLResponse,
                response.statusCode == 200,
                let data = data,
                let json = try? JSONSerialization.jsonObject(with: data, options: []) as? [String : Any],
                let clientSecret = json["clientSecret"] as? String,
                let publishableKey = json["publishableKey"] as? String
            else {
                let message = error?.localizedDescription ?? "Failed to decode response from server."
                self?.displayAlert(title: "Error loading page", message: message)
                return
            }

            print("Created PaymentIntent")
            print(clientSecret)
            print(publishableKey)
            self?.paymentIntentClientSecret = clientSecret
            StripeAPI.defaultPublishableKey = publishableKey
            DispatchQueue.main.async {
                self?.payButton.isEnabled = true
            }
        })

        task.resume()
    }

    func displayAlert(title: String, message: String? = nil) {
        DispatchQueue.main.async {
            let alertController = UIAlertController(title: title, message: message, preferredStyle: .alert)
            alertController.addAction(UIAlertAction(title: "OK", style: .default))
            self.present(alertController, animated: true)
        }
    }

    @objc
    func pay() {
        guard let paymentIntentClientSecret = self.paymentIntentClientSecret else {
            return
        }

        var configuration = PaymentSheet.Configuration()
        configuration.merchantDisplayName = "Example, Inc."

        let paymentSheet = PaymentSheet(
            paymentIntentClientSecret: paymentIntentClientSecret,
            configuration: configuration)

        paymentSheet.present(from: self) { [weak self] (paymentResult) in
            switch paymentResult {
            case .completed:
                self?.displayAlert(title: "Payment complete!")
            case .canceled:
                print("Payment canceled!")
            case .failed(let error):
                self?.displayAlert(title: "Payment failed", message: error.localizedDescription)
            }
        }
    }
    
    @objc
    func address() {
        self.addressViewController = AddressViewController(configuration: addressConfiguration, delegate: self)
        let navigationController = UINavigationController(rootViewController: addressViewController!)

        present(navigationController, animated: true)
    }
}

extension CheckoutViewController: AddressViewControllerDelegate {
    func addressViewControllerDidFinish(_ addressViewController: AddressViewController, with address: AddressViewController.AddressDetails?) {
        addressViewController.dismiss(animated: true)
        self.addressDetails = address
    }
}

My question is that, now that I can collect address from user, the “collected address” seems to be stored in nowhere and is not called in anyway as the user make payment. Such a “collected address” is of no use to me if for each payment I cannot refer to where I am going to ship my product. So how can I modify the code to somehow link the two together and be able to refer to it in Stripe backend? The Stripe documentation does not seem to provide any reference code to that.

Please help… Thanks!