One of my reasons for WhoisCart was that it could be easily extended to cope with other registrars and panels. The developer wrote an interface to the Helm API for me a year or so ago and I recently wrote an interface to the DotNetPanel API. I've also written a hook into my UK domain registrar. CE did seem OK, but not as flexible as WhoisCart in my opinion.
He cranked out the HELM HAPI very quickly. I gave him access to a Helm reseller account and the HAPI was ready later that day, (or it might have been the next day, but very fast anyway)