If you are using cPanel/WHM you can set a single domain ssl certificate as the shared certificate for that server (I'm sure you can do this with other control panels, but I only have experience with WHM). In this case, there is no need to purchase a wild cart cert. If you chose to do it this way, you could use a domain specifically for the shared ssl (the domain would need a dedicated ip address) and your customers could use it in the following fashion:
https://www.domain.com/~username/cart.html. They would just have to make sure they created any links to secure areas of their site in this way. However, this method should really only be used for a secure sign up or login that is unrelated to commerce. If a transaction will be taking place, you should recommend to your customers that they purchase their own private ssl certificate and upgrade to a dedicated ip address