Domain Reseller
 

I want to offer my customers a way to register a domain name through my site.

Are there any domain name resellers companies that you use?

go with enom

Are there any cost involved?

I am just getting started with my mainstream development company.

Looks like it
http://www.enom.com/instant-reseller/default.aspx

I just want to be able to add domain name registration to my website.

I don't need it to be branded.

Here are your options
http://www.enom.com/resellers/default.aspx

They have an API you can simply integrate
http://www.enom.com/resellers/api-reseller.aspx