WP WooCommerce Payment integration issues

We have a custom created plugin on our website, www.ciliciabookstore.com. The plugin which is a payment gateway add-on for WooCommerce, probably have 2 issues. please find below the notes sent by the payment gateway providers. You have to check the errors and provide me a solution to fix them. 1. Amount is stored in a hidden field on the form; it is easy to modify it before it’s encrypted and sent to our gateway. Any malicious user will end up buying any item for 0.01. The amount and confidential information must be retrieved from a secure server side. 2. The page receiving the response page must make sure to validate the securehash before approving the order. I have modified the value of txnresponsecode from 7 to 0 (approved) in the URL and the order was accepted. This is because hash validation wasn’t properly handled.