To answer your last question first, yes you need to open a ticket with the help desk to get them to install the certificate (it does incur a fee).
If you have you own certificate the links to your cart will be:
>https://yourdomain.ext/path-to-cart-or-file
Using the shared certificate the links to your cart or files will be:
>https://servername/~username/path-to-cart-or-file