Fix PayPal express function in Zencart

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Job Description is my Zencart powered site. It has been working flawlessly for a year and around 8/29/2013 the customer checkout with PayPal began to halt with a 404 page not found error following the customer clicking the final "confirm order" box in the check out. Please try it if you like. I put in a test customer name and password for your use. Use ""; as the user and the password is "testtest". I spent yesterday and today with my web provider and PayPal trying to locate problem but they couldn't find it. I was told that it is just a simple file problem in Zencart. I need a fixed price quote and It needs to be done right away since my customers can't make a purchase (I have to PayPal invoice them). DO NOT QUOTE IF YOU CAN NOT COMPLETE THIS SUCESSFULLY AND QUICKLY. SORRY ABOUT THAT, BUT, I REALLY NEED THIS DONE RIGHT AND FAST! BUDGET BELOW IS FOR REF (WANTED TO USE THE WORD "QUOTE")