A group of Uber drivers in New York City is campaigning the app to add a tipping option, insisting that Uber's refusal to include such a feature is the primary grievance among their 35,000 members. The news comes several months after Uber, which was recently valued at $62.5 billion, slashed its NYC trip fares by 15%.

"Many passengers are unaware that tipping Uber drivers is permitted or expected," said Uber driver Ronny Paulino in a statement. "Tipping makes a huge difference for drivers. When I drove for a service that allowed in-app tipping, those tips were enough to pay my family's grocery bill for the week."

To get the word out, Uber drivers will start posting stickers in their cars that read, "Tips for service are appreciated!" A social media campaign and pro-tipping petition are expected later in the summer.

Uber, meanwhile, is showing no signs of budging. "On Uber, tips are not included, nor are they expected or required," stated a spokeswoman for the app on Monday. "Riders tell us that one of the things they like most about Uber is that it’s hassle-free. Riders are free to offer tips and drivers are welcome to accept them, as has always been our policy."

The drivers' push for a tipping option is spearheaded by the newly-formed Independent Drivers Guild (IDG), an association of NYC Uber drivers that, unlike a traditional union, does not give its members the right to collectively bargain for a contract. Uber formally recognized the IDG in May, promising to meet with its leadership once a month.

Because Uber drivers in New York City and across the country are classified as independent contractors rather than employees, the startup has no legal obligation to bargain.

Under the IDG's agreement with Uber, member drivers now have access to discounted legal services, as well as life and supplemental disability insurance, roadside assistance, and a website consolidating driver "assistance and resources," according to the app. However, driver fares, benefits and protections are still set by the company.

Reached for comment shortly after the IDG was formed this spring, one driver expressed skepticism, speaking anonymously for fear of retribution from his employer.

"Usually corporations are against unions, because they work on behalf of the workers for higher wages and better conditions," he said. "When you have Uber saying, 'Oh yeah, we came to an agreement that you guys are going to be represented...' it's not necessarily working for the people. They're looking for a mediator to pipe down the protests."

Gothamist editor Jen Carlson delved into the question of voluntary Uber tipping in February, soon after the fare cuts were announced in NYC.

"When the service was first introduced it was more expensive, and offered up mostly black cars—at that time there was a pretty firm 'no tipping' policy, and it made sense," she recalled. "The problem is this no tipping policy seems to have stuck even though the service has come more in line with the common cab. But you are tipping your cab driver, so why wouldn't you tip your Uber driver?"

If you're interested in tipping but not sure where to start, we suggest: 20% for longer trips, i.e. the airport; 20% for complicated trips with multiple stops; $1-2 for shorter drives, say, under five minutes.

And if you're looking to support the interests of full-time drivers, simply turn off your app during any future organized strike.