You can use WHMCS for whatever you want. No need to be only hosting products. We don't use WHMCS, we prefer Blesta, however we have clients from our design company, that don't host with us, and we keep using this ticket system almost in a daily basis for discussing several questions, or open tickets for support not related with hosting :-)
__________________ Welcome to FloXera SSD Web Hosting :. Managed Reseller :. Managed VPS
DDoS Protection :. R1Soft Backups :. Softaculous :. Let's Encrypt
Available Locations :. Germany Europe :. L.A. USA (Asia Optimized IP)
Tawk.to is also an option that you can consider. It is free and has option of live chat / ticket system along with email piping. If you need unbranded version (without powered by tawk.to line), it is available at $15/m additional.