automate website payment process with paypal

Hi, Currently, when a user pays us through paypal, we have to manually update his account through our admin panel. We sell credits for use on our website. We want this to be an automated process. Buyer pays -> Their account is topped up immediately (appropriate database value changed) This process must be secure, there cannot be a way for people to trick the system into thinking they have paid. Must be reasonably easy to change prices. There are only 6 options for people to purchase. The system must send us an email alerting us to payment, containing the username, paypal email address, and amount purchased. Our existing manual system must remain functional. Money in escrow, half up front, half upon completion and testing. Thanks!