If they already own/registered all the other tlds for the word/term. I certainly wouldn't insult their intelligence by going down the road of explaining the advantages to having the .com. Keep this 'follow-up' simple and sweet . Thank them for the email address, then move straight to your expectation of a sale price, mention as many payment options as you can accommodate. Then ask them to share their thoughts.
I have a couple of similar situations where potential buyers have all the other tlds (and me the .com) It certainly doesn't make it any easier. Sometimes they hand register all the other tlds then don't expect to pay much more for the Prized .com.
In fact I think registering all the other tlds sometimes is a sign that they are not thinking straight. ie better to try and obtain your .Com domain before showing their hand by going for all the other extensions. So respond with all this in mind.
Obviously you should also be keeping in-mind, If any of the other tlds they own are live, their business type, and even how recent their registrations are. If they registered the other tlds years ago, and haven't approached you in the past, then they are in no-rush and are probably just keeping an eye on any possible drop of the .com.
Finally if they registered all the others recently, then your in the best sales position your probably ever going to be - then I'd be playing a little bit more hard-ball