Hi,
When I looked at your code, I ddin't see where you're pulling the users or payers email from the database, you never did anything with it.
This code:
//Below is what I am doing to send email
$query="SELECT id, uid, email, firstname, lastname from slist where id='$id' and uid='uid'";
$Subject = "Test";
$content = "test this out";
mail ("$payer_email", "$Subject", "$content");
[/B]Where did you store off the row from your select query?[/B]
After you defined your sql statement where did you 'run' the sql?
Where did you connect to the database, and run your query. You have to do all that to retrive the row back for that person or payor.
For your other question... .there is actually a 'from' parameter in PHP's mail() function
http://us4.php.net/manual/en/ref.mail.php
Hope that helps!!