osCommerce Platform Enhancement

We created a balance system, we need it to be developed at checkout in the online store by clicking on the use balance button, checking in PHP and Javascript if there is a balance, if the balance is valid or has not expired, if everything is ok to add the balance in the totals to deduct from the total.

MVC framework
OOP Classes and Functions
PHP, JavaScript, Jquery, MySQL

It has 2 types of checkouts
- One-page checkout
- Checkout on more pages
Need to be implemented at both checkouts

Requirements:
Describe and document all the changed files and lines.

All files related to checkout were sent, including the following files:
checkout.php
checkout_confirmation.php
checkout_payment.php
checkout_process.php
includes/checkout/checkout.js
includes/classes/order.php
includes/classes/onepage_checkout.php
includes/functions/html_output.php
includes/languages/
includes/modules/order_total/ot_balance.php

When clicking the Use balance button in the checkout.php and checkout_payment.php files, the javascript in the includes/checkout/checkout.js file and in the OOP in the includes/classes/order.php, includes/classes/onepage_checkout.php and includes files are activated. /modules/order_total/ot_balance.php go to the backend and validate the balance as per the attached image and shown in the project, then show the balance in includes/modules/order_total/ot_balance.php in which the use of the balance would be displayed. The balance must appear in the order classes when validated and used.

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