I used the following fields:
- Invoice number (This is not required)
Here is the script which is called on form process:
$item_name = 'Payment on Invoice Number'.$_POST['form']['InvoiceNumber'];
$currency_code = 'USD';
$amount = $_POST['form']['Amount'];
EDIT: I realized that the email notifications were not sending using this method. If you also want the email notifications to go out, move this PHP code to the "call AFTER form process" box in RSForm.
BONUS READING: Connecting RSForm and Paypal using recurring payments