When you have sold a large system, you have probably covered lots of options and further selling is not likely needed.

It's the small upgrades and troubleshooting calls where you should be ready with upgrade paths. Sonos always deserves a mention, and I keep some basic gear on the truck. I keep a few URC remotes when one is not currently used at all or an old text remote is on the premises. Network gear.

You don't have to hard sell, nor should you. But if you educate and there are good reasons behind the gear you are carrying, there will be add-ons.

To not be prepared for it is not good business!