Generally, with Fantastico, installation is a one click process.

If you have Fantastico on your servers, and the forum software enabled for Fantastico, simply tell your customer yes.

It is then really up to them to install the software.

Now, if you really want to hit a market, offer to install it for them...keeping in mind that this could be pretty time consuming depending on how many people ask you to do this.

The thing to stay away from is opening the door to allowing your customer to think you'll provide technical support for the application, outside of the realm of hosting it.

In other words, if your server environment supports the software, but the software still doesn't work, then it's not your problem.

If you fall into the trap of allowing them to milk you for customer support for the application itself, you will do nothing but this, and the customer will come to you with a ton of things that you can't do anything about, and ultimately get angry with you when you can't.