according to the error, the problem is occuring on line 6 of the cart.php page:
output started at /home/isaacsgr/public_html/meganisaacs/test2/cart.php:6
The headers already sent error means there is output to the page beforer the header function is called, that output is at line 6.
line 1 - 6 0f your cart.php page are:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>
<?php
The error is caused by the blank space at the end of line 4 and the blank line at line 5.
change the code at lines 1 - 6 to:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>
<?php