Flask CORS issue with Flutter web?

Question

I am trying to integrate Stripe Checkout in a flutter web app. Every time I send the POST request, I keep getting the CORS blocked error. I have been dealing with this problem for days now. I have tried every solution I could find online. I have tried adding Flask_Cors, cross_origin. I have tried adding the headers. Nothing seems to work. I am pretty new to backend stuff so I am not sure whats going on. Here is my Flutter code:

Future checkout(double amount) async {
    var url = Uri.parse('https://spaceshuttleparking-checkout.herokuapp.com/create-checkout-session');
    final response = await http.post(
        headers:{
          "Accept": "application/json",
          "Access-Control-Allow-Origin": "*",
          "Access-Origin-Allow-Methods": "GET, DELETE, HEAD, OPTIONS, POST"
        },
        url,
        body: json.encode(
            {
              'parking':{
                'name':'Parking Space',
                'amount':amount,
              }
            }
        )).then((value) {
      print(value.body);
      print(value.statusCode);
      print(value.request);
    });
  }

Here is the server code(hosted on Heroku):

import json
import os
import stripe
from flask import Flask, render_template, jsonify, request, redirect
from flask_cors import CORS, cross_origin

# This is your test secret API key.
stripe.api_key = 'sk_test_51LfcOSB1SRO7vxqKhIxQE6vTmXKkYTBJoqXGNHYCwI0nCosmQpVG8MJb7TJala9hjOUZIDh8A9XPgJ1FYTETJBO700uWTUBSbG'


app = Flask(__name__, static_folder='public',static_url_path='', template_folder='public') 
app.debug = True
CORS(app, resources={r"*": {"origins": "*"}})


app.config['CORS_HEADERS'] = 'Content-Type'


def calculate_order_amount(items):
    # Replace this constant with a calculation of the order's amount
    # Calculate the order total on the server to prevent
    # people from directly manipulating the amount on the client
    return 1400


@app.route('/create-checkout-session', methods=['POST', 'GET'])
@cross_origin(supports_credentials=True)

def create_checkout_session():

    request_data = request.data
    request_data = json.loads(request_data.decode('utf-8'))
    try:
        parking = request_data['parking']
        session = stripe.checkout.Session.create(
            line_items = [
                {
                    'price_data': {
                        'currency':'aud',
                        'product_data': {
                            'name': parking['name'],
                        },
                        'unit_amount': parking['amount'],
                    },
                    'quantity':1,
                }
            ],
            mode='payment',
            success_url='https://spaceshuttleparking-checkout.herokuapp.com/success',
            cancel_url='https://spaceshuttleparking-checkout.herokuapp.com/cancel'   
        )
        return redirect(session.url, code=303)
    except Exception as e:
        return jsonify(error=str(e)), 403

if __name__ == '__main__':
    app.run(port=4242)

Any help is greatly appreciated.