how to use “update_usermeta()” in an independent PHP file inside WP folder

I am going to create an independent PHP file in my public_html folder, which will callback by Google Checkout or Paypal and receive payment notification. I want to add user info into Database when it did.

As I know, I have to use function “update_usermeta()” to add those user information. But the thing is, how can I use it in an file that does not belong to WordPress blog system. Which file should I include or requrie?
Can I write plugin for it?

This PHP file is inside the WordPress folder, with wp-login …