"Re-declare class" fatal error during payment process.
During submit, for example paid listing, the script calls payItem() function in ia.front.plan.php
In that function script tries to initiate item class to continue "postPayment" process.
$iaItem = $this->iaCore->factoryPackage($class_name, $iaItems->getPackageByItem($item));
But if item class is already declared - we got fatal error.
The bug could be checked by submitting paid/sponsored listing from Admin Panel.