Your customers can register and renew domains by CE. But they cannot manage them. They can register, renew and manage domains by AWBS. AWBS supports more features. We have CE and AWBS license and we tryed them. You will need many times to customization of these softwares if you buy one. Because there are a lot of files and the size of these softwares are very big. Good luck.
All the above mentioned systems like modernbill, AWBS, etc. cost more than 100$, if you are looking for temporary solution, i would recommend you to checkout whois.cart. supports many domain registrars and also integrates your whm / cpanel to create hosting accounts once they are paid for automatically.
Also whois.cart now has a add-on to add a support system to it.
www.awbs.com is probably the most "feature complete" hosting billing management system that includes domain registration.
Since it started off as a domain management system first, it has a lot of features and works with all the major domain reseller registrars.
We are using it and it is very good. (Although it is ugly and the template system is bad.)
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AWBS is definitely well integrated with Enom, Directi and eGold. You can offer most of Enom domain services via AWBS. If you are only selling basic domain registration, majority of the client billing software does that.
It has the most advance domain registration and management functions. Gotta agree with mrzippy that their templates are a really pain to integrate.