Native Android IAB

The entry point for this process is after you get a valid purchase notification through
onPurchasesUpdated

@Override public void onPurchasesUpdated(int responseCode, 
                                         List<Purchase> purchases) {
  Purchase purhcase = purchases.get(0);
  String sku = purchase.getSku();
  String token = purchase.getToken();

  Log.d(TAG, "purchase sku:" + sku + ", token:" + token);
}

The sku and the token can be directly obtained from the Purchase object. Having them, you can fire a request to your server with the required information:

Package_name
Sku
Purchase_token

Sample code:

HttpURLConnection conn = null;
try {
  URL url = new URL(baseHost + "/purchase/" + applicationPackageName + "/check");
  conn = (HttpURLConnection) url.openConnection();
  conn.setRequestMethod("POST");
  conn.setRequestProperty("Content-Type", "application/json;charset=UTF-8");
  conn.setRequestProperty("Accept", "application/json");
  conn.setDoOutput(true);
  conn.setDoInput(true);
  JSONObject jsonParam = new JSONObject();
  jsonParam.put("token", token);
  jsonParam.put("product", sku);
  Log.i("JSON", jsonParam.toString());
  DataOutputStream os = new DataOutputStream(conn.getOutputStream());
  os.writeBytes(jsonParam.toString());
  os.flush();
  os.close();
  int responseCode = conn.getResponseCode();
  if (responseCode == 200) {
    PurchaseVerificationResponse purchaseVerificationResponse =
      gson.fromJson(new InputStreamReader(conn.getInputStream()),
                    PurchaseVerificationResponse.class);
    Log.i(TAG, purchaseVerificationResponse.toString());
    listener.onPurchaseValidationResult(sku, token, purchaseVerificationResponse.getStatus()
                                        == PurchaseVerificationResponse.Status.SUCCESS);
  } else {
    listener.onPurchaseValidationError(sku, token, new Exception(
      "Response code: " + responseCode + "\n" + "Message: " + conn.getResponseMessage()));
  }
} catch (Exception e) {
  listener.onPurchaseValidationError(sku, token, e);
  e.printStackTrace();
} finally {
  if (conn != null) {
    conn.disconnect();
}

📘
You can check https://github.com/Catappult/appcoins-iab-sample for a fully working sample.

Using Unity IAB

The entry point for this process is after you get a valid purchase notification through OnPurchaseSuccessful

public void OnPurchaseSuccessful(Product p) {
        Debug.Log("purchase successful with receipt: " + p.receipt);
}

receipt is a json string that you can parse and get the purchase token from.

Having the token you can fire a request to your server with the required information:

Package_name
Sku
Purchase_token

Sample code:

Debug.Log("payload token: " + purchaseToken + "\n payload store specific id: " + sku);
if (purchaseToken != "" && sku != "")
{
  WWWForm form = new WWWForm();
  form.AddField("token", purchaseToken);
  form.AddField("product", sku);

  //Sample url, your server url should go here!
  string url = "https://validators.aptoide.com/purchase/" + packageName + "/check";

  Debug.Log("Going to post:\n" + form + "\n to " + url);

  using (UnityWebRequest www = UnityWebRequest.Post(url, form))
  {
    yield return www.SendWebRequest();

    if (www.isNetworkError || www.isHttpError)
    {
      Debug.Log(www.error);
      //deal with error
    }
    else
    {
      //Need to check response
      string response = System.Text.Encoding.UTF8.GetString(www.downloadHandler.data);

      //Parse response code from json response  
      m = Regex.Match(response, "status\": \"(\\w+)");
      string status = "";

      if (m.Groups.Count > 0)
      {
        status = m.Groups[1].Captures[0].Value;
      }

      if (status == "SUCCESS") {
        Debug.Log("Purchase verified!");
        GameManager.Instance.OnFullTrajectoryPurchased();
        _purchaseBtn.SetActive(false);
      } else
      {
        Debug.LogError("Failed to validate purchase");
      }
    }
  }
}

📘
You can check https://github.com/Aptoide/appcoins-mwc-game/ for a fully working sample.