E-junkie offers a lot of features (
http://www.e-junkie.com/ej/features.php) which you might or might not need. Some of the key ones creating discount codes, managing inventory, affiliate management and providing a slick shopping cart which enables you to accept Google Checkout as well.
But of course, you understand the nature and requirements your business better than anyone else so only you can tell whether that additional cost is justified or not.
Feel free to contact if you have any more questions.