Contact your host. They should do the install for you - most do it without a fee. If they are running a control panel and have the options enabled, you can do the installation directly through your control panel.

I've not heard of hosts restricting users to only purchase through their affiliate/recommended provider before. Seems odd to me.

As for installing the SSL Certificate yourself, if your host has cPanel and has enabled the options, end clients CAN install their own certificates without any help from the host themselves. This requires that the host has previously enabled the option for you however. This is something that we offer on our hosting packages, but not all hosts do this. Most like to provide it as a service to their customers.