So personally I'm using Box Billing (Pro) and I love it as its easy to use and pretty decent built. I think they need to offer extentions again and fix a couple bugs in the admin panel with the product promotions. I used Clientexce and then switched to Box Billing as I got a free premium license with my reseller hosting. I used THT before and found it not working the best for me as it didn't send a welcome email with the cPanel and name servers