Be sure to ask what types of payment they want to accept. Credit cards? Which ones? E-checks? Paypal? Google checkout? Are card numbers being stored on the payment gateway? If not are you PCI compliant? Do you already have SSL? If not, who's getting the SSL cert? How do you want to handle refunds? Are cancellations pro-rated? Do you want to support affiliates? Do you want to have discount codes? Do subscriptions have setup fees?
