Upselling that people find is annoying: "Can I also get you a croissant with that?"

Upselling that feels like a recommendation: "Have you tried it with X syrup yet? It's amazing" / "Would you like to add an extra shot of espresso this morning?" / "Do you want to make that a trenta, our extra large size?"

**As long as it feels like you're trying to make their current order better, people are likely to take it.** If it sounds like "also order this random thing," it feels pushy

Edit: for your specific upsells, I recommend: "Do you want to make that a venti?" instead of "what size?", and depending on how much you want to push it, start with "Thanks for stopping at Starbucks, can we get you started with a [name of base] lemonade refresher to beat the heat?" or *customer orders refresher* "And would you like to try that made with lemonade today?

I get why you would feel bad, but part of why baristas are supposed to be knowledgeable about drinks is to help people find a drink they'd like best. Sometimes making a drink taste better means adding something, so you're just helping them get a better flavour or stay more hydrated