For domains, you can sign up to be a reseller with a number of registrars. I use resellerclub because they are one of the few who don't charge a monthly fee for this and they have a nice API. They take all of your billing info, and allow you to enter your clients' info. They charge you one price and you charge the client another, and they can handle all communication like renewal reminders, etc. directly with the client if you want. It's better to have the domain in the client's name, not yours.
That's some good advice. Usually if the projects' price is steep, I'd suggest the client to pick for themselves and foot the bill. Even though it costs the same, it doesn't seem like it's your expense.
