best third party payment system?

Hi everyone,
I didn't know where to turn for advice so I came here. I'm looking for a third party company to help customers at my site make their purchases. After a purchase, I need the company to provide some kind of service to make a mysql command to tell my database to update the number of credits a person has (a user will go to paypal to purchase credits and the mysql database should immediately update this information to reflect how many credits a user has).

Does anyone know of a way to do this through php/mysql/paypal, or is there a good, cheap third party solution?

Thanks in advance,

Julian

On paypal where you make a buy now button couldnt you just create the target page (once the user has completed payment) have a MySQL command on it?

Well I could, but then couldn't anyone who knew the payment-successful URL go to the URL and have the mysql command add credits to that person's account even if that person didn't buy anything?