You could try making the following edit to /includes/modules/payment/paypaldp.php, at line 539:
Replace:
Code:
// unused at present:
// $options['CUSTOM'] = '';
// $options['INVNUM'] = '';
// $options['DESC'] = '';
with this:
Code:
// send the store name as transaction identifier, to help distinguish payments between multiple stores:
$options['INVNUM'] = STORE_NAME; // (cannot send actual invoice number because it's not assigned until after payment is completed)
// $options['CUSTOM'] = ''; // reserved for internal use in next version of Zen Cart:
// $options['DESC'] = ''; // reserved for internal use in next version of Zen Cart:
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