Need Smarty & PHP Coder ASAP to Debug a Simple Bug in PHP Cart

You must know Smarty, PHP, Ajax to work on this cart. We have a very old version of a PHP cart. There is a small bug that's preventing from customers from checking out. This only happened on a few visitors. For some reason during the shipping and billing form, there is a bug that pops up for certain visitors saying that invalid email address no matter what the email address is, the customer put over 5 email addresses and still had the same problem. We tried clearing the cookies, logging out of her session, nothing is working. I believe we need to make the email address field to be ignored and not mandatory in the checkout field. so no matter what's filled out in the email field, it'll let the customer pass through and checkout. The cart is heavily custom coded, so need to make sure it doesn't interfere with all the other coding. The version of the cart is over 8 years old. Cart uses PHP and Ajax. Budget is $20 for this bug fix. Should be a pretty simple fix.