I use WHMCS to sell SSL certificates. I use both the SSL modules that are available for WHMCS and also just to receive the orders and then handle the orders manual.
I miss a better opportunity to configure how long the SSL certificate is valid. Today you should set it up as a configurable option with a one-time payment of $ 0 - it gives a very misleading impression when people are surfing the catalog.
I also sell SSL certificates in Denmark in Danish where I use the e-commerce system Magento. Here I show the base price in the catalog which is a certificate valid for one year - and a configurable option with number of years (and how much more the price will be) – for instance 1 year – 2 years (+ $20) – 3 years (+ $35). It would be really nice to have this feature in WHMCS since it gives a more correct impression.