In addition to the other module, 8Dweb sponsored development for a sub module to the excellent Modules Garden DNS Manager.
There were two options, 1) change and extend the Interworx-Extended Module to handle DNS for Interworx OR 2) sponsor a sub-module to let DNS Manager handle it (and thus have regular updates).
We chose the latter.
If you are interested in using WHMCS as the front-end to your Interworx Sales, but need better options for your customers, check out the DNS Manager with Interworx SubModule.
Simply contact Modulesgarden.
Please do not contact 8Dweb, we have no further role than the sponsorship.
8Dweb has been frustrated for years at the lack of decent customer/billing portal support for Interworx. We are simply happy a 3rd party has come along with some excellent integration tools.