This question is for both ModernBill and H-Sphere's billing system. Once a customer registers a domain through one of these systems, how does the customer manage the domain? Meaning how do they set name servers or change contact info, etc?
What I do know is that if a customer buys from you compared to some one else, they have a reason. And if they leave over price because they need to make a changes to their domain name contact information (from our experience, and extremely rare event to change domain name contact information), then the selling process should be re-evaluated (my opinion only).
Not wanting to take this too far off topic, but enom does provide a reseller interface option. Registry Rocket allows you to use a branded (although limited), vanilla interface for both administrating and signing up domains. You can even set up domain forwarding so that you can use access.yourdomain.com for the url.