WHMAP lacks flexibility and funcitionality. Throwing this one out on the table for you all; I used WhoisCart....ugh, don't even bother. They boast alot for their price, the screenshots look good, the feedback looks good, you buy, you get it and bleh just horrible.
I just ordered ClientExec so I am going to try that. Haven't tried ModernBill yet so I can't really comment on that. However, I do recommend using FraudGate in conjunction to your panel (i.e. ModernBill, ect.)