Debug osCommerce/PayPal

I have an osCommerce Store that has been working fine for many years. Now there is a problem. I'm not sure why. I have had three coders work on it, unsuccessfully, and with each coder the problem changes. Currently this is a summary of the problem: After the customer enters their Payment Information in the Checkout Process of the Shopping Cart they are taken to the PayPal login page. 1. Instead, they should be shown the Summary of the order, including shipping and taxes (if included). They should be allowed to confirm the order or change the amounts/quantities before being taken to the PayPal login. After logging into PayPal and clicking the Pay Now button, 2. the customer should receive a confirmation email with details of the order, 3. I should receive an order notification and 4. PayPal should send me notification of the transaction. None of that happens. Instead I am taken to the “What’s in my cart?” page which says it’s empty, but if I login again, my previous purchases are shown in the cart. As mentioned, I have had three coders work on this problem. None have succeeded. I need someone who is very familiar with osCommerce and PayPal to resolve this issue within a week from the time started. If it is working as it should I will pay promptly. If is it not fixed after a week I will not pay for any work done, and will request cancellation of this contract so that I can hire someone else. Please do not apply for this job unless you are really good at debugging and believe that you can fix it.