If you buy a $4.50 coffee with a $5 bill and don't put the fifty cents in the tip jar, the worst that happens is that the barista looks at you uncomfortably for a second or two. Then you get on with your day. If they rate you along with other baristas (or coffee shops) you'll have a reputation before you set foot through the door.
Growing up in America, I didn't understand how absurd our tipping culture is until I visited Europe. There, wait staff don't bother you every five minutes to "ask how you're doing," you call them over when you need them, and if at the end of the meal you enjoyed your service, you can leave a few euros as an extra, not a semblance of a living wage because the employer refuses to pay it.
Still, if it's really that important to you to tip your Uber driver, use cash.