Write some Software

Hello, I sell digital products off my website, Someone tricked my current Paypal IPN delivering system to send them the product they bought, which is meant to be $54.99, but they changed the checkout price to $0,01 and they received the product because my website has an automated delivery system. I need the IPN code to be amended so it checks for fraud like this, so it checks that the purchase amount is the same as it is on my website. So if they try to change it to $0.01, they will receive nothing., and the transaction won't come up as successful.