Session value is not getting after payment completion - asp.net-mvc

Question

I am using paytabs(a payment gateway like paypal) in my mvc project.In that
when when I call create-paypage-api-call it will go to paytab website and complete
the payment section and will return back to my page.
return back url is an action method in same controller and that url i have to pass as a parameter when calling create-paypage-api-call.
before calling it I am saving customerID in session. But I am not able to get the session
value in the retrurnpage.
Here is my code
This is my payment gateway calling actionresult
public ActionResult paymentcall()
{
//my logic part here let it be int customerid = 123
session[id] = customerid;
//create-paypage-api-call starts here
request = (HttpWebRequest)WebRequest.Create("https://www.paytabs.com/apiv2/create_pay_page");
request.Method = "POST";
formContent = "merchant_email=bb#gmail.com&amount=333"// formcontent will contain some more parameters that i wont mention here
formContent += "&return_url=" + "http://example.com/mycontroller/TheReturnPage"; // this is the returnpage after payment
byteArray = Encoding.UTF8.GetBytes(formContent);
request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = byteArray.Length;
dataStream = request.GetRequestStream();
dataStream.Write(byteArray, 0, byteArray.Length);
dataStream.Close();
response = request.GetResponse();
dataStream = response.GetResponseStream();
reader = new StreamReader(dataStream);
responseFromServer = HttpUtility.UrlDecode(reader.ReadToEnd());
PTResp = new PromoPayTabsMakePaymentResponse();
PTResp = JsonConvert.DeserializeObject<PromoPayTabsMakePaymentResponse>(responseFromServer);
System.Web.HttpContext.Current.Response.Redirect(PTResp.payment_url);
}
when call this it will go to the paytabs website and complete the payment there like entering
card details cvv . after payment it is successfully returning back to TheReturnPage actionmethod
public ActionResult TheReturnPage()
{
int aa = Convert.ToInt32(Session["id"]);
}
but in returnpage i am getting session value zero.
I tride redirecting from paymentcall action just after creating session. At that time it is getting.Session value is is not getting when it goes to paytabs page and returning back from there.
(paytabs page is https and my website is http. I dont know whether this info is needed. Just mentioning)

Answer 1

I am guessing because you redirect to paytabs and when it return page back to you, it is not the same session as the one you had with your customer (because it is from paytabs to you now)
The only thing I can think of is put a transaction id or something which you can identify on the return url, such that:
formContent += "&return_url=" + "http://example.com/mycontroller/TheReturnPage?id=" + someId
then use that someId to retrieve customer info from a database or some external storage.