I have used cheap-domainregistration.com for about five years now.
Only once I had a problem with them, I called customer service on Sunday, I got someone in the phone pretty quickly, and my problem solved in about 30 minutes.
That is correct if you are going to be processing customer credit cards, which I would not recommend. Instead I would use PayPal for it.
Check feedback from customers about customer service for those companies.
I use Lunarpages and I am very happy, so far.
Sometimes it is not just the price what you have to look for. I know we all do it. But a little peace of mind is good too.
Whatever choice you make, try to get a little peace of mind along the good deal.
Good luck