I have to disagree. I would stay as far away from 2CheckOut as possible. While there support is decent, they almost always take at least 24 hours to determine if an order is fraudulent or not. This means that you could have already ordered a domain and activated a new client's account, just to have 2CheckOut turn around a day later a say the order is fraudulent and you will not be receiving the payment. This means you are out the $8 or so it cost you for the domain. You could just have orders stay in a pending state until you receive the fraud review, but many clients would not like that.

Since you are a ModernBill user, I recommend you go to http://www.modernauthorize.com and get the $49 setup fee BluePay merchant account. The setup fee is the same as 2CheckOut and the transaction fees are actually lower. You will also need to get an SSL certificate if you choose to get a real merchant account such as this.