Sorry, but putting in a commsion for selling crap the customer doesnt need is just an easy way to turn your tech into a parts changer and get your business a reputation for being a hustler. We dont upsell stuff the job doesnt need. If the customer complains about turning the thermostat to different temperatures all day (while youre there fixing something else) and they have a T87, then absolutely suggest a programmable. But dont go in there with the mindset you are going to sell them the world, and dont make your techs do it either!
Someone mentioned a fan motor when replacing a capacitor. thats not upselling in my opinion. Thats being thorough and making sure the unit is fixed for good. If you fix it once and its done, then you will be referred and you will get more business. If you want to take the customer for all theyre worth, that will be the last time they ever speak your name without putting some choice words in front of it.
Look at each customer as your own parents. Would you do to them what youre expecting your workers to do? NO? Then why are you doing it to someone elses?