ProSites and WHMCS landing pages

I am using WHMCS as my provisioning/purchasing/account management software and I'm using pro sites as a way to handle the level of access a user gets upon registering a site.

The problem that I'm having is that ProSites still wants to prompt people for purchases, etc. For instance if a person goes to xyz.gracesites.com it takes them to the prosites landing page and prompts them to login if they aren't logged in or sign up for a prosites plan if they are. I'd really rather it direct them to my WHMCS client page instead.

Is there an easier way to handle premium themes/plugins, or is ProSites pretty much it? And if that's all I have , how do I get ProSites to hand everything over to WHMCS?