OK, I figured out what was wrong with some help from Allyson - Apparently the code for config.inc.php and process.php aren't formatted correctly. Am I the only one here who has setup the newest version of FX Cart? Well, in any case, here's what needs to be changed:
In config.inc.php, all instances of these:
$paypal[business]="user@yourplace.org";
Should look like this:
$paypal['business']="user@yourplace.org";
and all instances of these:
$paypal[firstname]="_POST[firstname]";
Should look like this:
$paypal['firstname']='$_POST[firstname]';
Also, in process.php, this line:
<form method="post" name="paypal_form" action="<?=$paypal[url] ?>">
Should be:
<form method="post" name="paypal_form" action="<?=$paypal['url'] ?>">
Hope that helps anyone else experiencing this problem.
Edit: This works with PHP 4 and 5, with IIS 6