Authorize.net Checkout Page DPM

Built a retail website from scratch, and and the pages and products are all over 90% done. Various pages lay out related products pulling the details from a MySQL database, and all link to a [login to view URL] page where pulling $_GET variables out of the query string allows it to render the proper product and properly link the add to cart button to the shopping cart. The shopping cart was also custom built from the ground up entirely in PHP, using a custom Cart object class that relies on sessions. The cart is fully functional, and works flawlessly in adding and removing products, working out shipping, and all those things. Currently the shopping cart has a checkout button that links to [login to view URL] under https, the SSL is fully installed and functional. The site is registered and set up through [login to view URL] with an active merchant account and all that. What's needed is the payment processing code from the checkout page. Anyone willing to throw in the JavaScript client side validation quick and cheap would be great, but all that's required is the server side validation and gateway connection. We need the form on the checkout page to take the customers email address, shipping info, billing info, credit card information, then pull the total out of the cart ($cart->total), process the customers credit card, then bring them back to the checkout form on failure, or bring them to a thank you/receipt page on success that will clear their cart ($cart->clear();), email them an invoice, email us the order, and charge their card. Any HTML/CSS duo required to tastefully integrate this checkout into the website can still be handled by us, we just need the back end PHP, and rawest HTML form structure compatible with the PHP script that will work. We're capable of doing all this on our own, but the checkout process with [login to view URL] is the one aspect we're least familiar with and have no experience in, and deadlines are getting tighter every day. So it'd be a life saver for any PHP developer with experience integrating [login to view URL] could just knock it out quickly for us while we focus on the other 5-10% of the site that still needs building/polishing/debugging. If you are not a seasoned PHP developer and/or have no experience with [login to view URL] you need not apply. We have Harvard computer science students working on this project and could figure it out too, we just don't have the time to, much less the time for some one else to figure it out. We need this done as soon as possible, and will provide any and all code/server access to whom ever takes on the job. Speed and budget are keys in this job. It doesn't have to be super pretty or graceful code, it just has to work. So please, anyone with experience with this, make some quick and easy money for than 1 days worth of work, you'll be a life saver.