This is the kind of thing that just cannot happen on a live site, I am more than likely going to get into legal trouble because of this and to say I'm upset about this is an understatement.
I setup a product on my website for ENOM SSL certificate. One of my customers bought one today for $29.99, the Plugin says $29.99, when they hit checkout it charged them $119.99 for 4 of them, this is unacceptable. If I do this through WHMCS without the bridge it charges them the right amount.
I've made a video about this, until it's resolved I have to take my entire store offline before I get sued for using this plugin.
This video shows me adding the product by using this plugin and getting charged 4x more for it, and then going through the non plugin mode directly through WHMCS where it charges the right amount.
I have no caching plugins enabled, the url to where this happens which you can test it by using amazon simple pay, and then just viewing the invoice, is at https://www.hbkhosting.com/hbk/hosting/cart.php/?gid=2
The video demonstrating this is at