Upsells that don't feel slimy
Real upselling is presenting what the customer would want if they knew what you know.
Offer the good, better, best tier
Three options on every estimate roughly doubles the rate at which customers pick the middle or top tier vs accepting a single price. Most customers want to feel like they chose.
Bundle the obvious add-on
The thing you'd recommend to your mother goes in the quote as a checked box, not an upsell pitch. They can decline. Most won't.
Stop selling once they pick
Once the customer chooses a tier, the upsell is over. Continuing to add on after they've decided is how trust breaks.
Take your version of this question further
This is one operator-tested angle on the question. Your shop, your size, your trade, and your team change the answer. Ask your specific version inside Ask a Shop Owner to get a response grounded in how owners like you actually handled it.