Conventional wisdom dictates that if you can afford to have food delivered, you should be able to afford the tip. Not always.



On Tuesday, a Reddit user named Jfastman uploaded a photo of a receipt for a pizza delivery that included 85 pizzas totaling $1,453. "My friend delivered 85 pizzas today and got a $10 tip," he wrote. The story kicked off a fiery debate prompting dozens to write comments such as "The delivery guy deserved more" and "Ten dollars is cheap!" Others insisted that no one could realistically expect to receive a 20 percent tip on such a large order.



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Tipping has been a hot topic lately and there's no shortage of water cooler news anecdotes to fan the flames. In October 2011, a waitress named Victoria Liss didn't receive a tip for the service she provided for a $28.98 meal, however she did receive a note that read, “P.S. You could stand to loose (sic) a few pounds.” In January, Reddit user Nickshambo posted a photo of a note a patron left his friend instead of a tip. Printed on paper, the note read: "As a direct result of Proposition 30 and President Obama's insistence that I pay "MY FAIR SHARE IN TAXES" I find that I must cut back on discretionary spending and gratuities. I wish it didn't have to be this way for both of us." And later that month, Reddit user Gateflan, a former waiter at Applebee's posted that a St. Louis-based pastor named Alois Bell reacted to his restaurant's "included tip" policy of 18 percent by writing a zero on the receipt's tip line and writing, "I give God 10 percent. Why do you get 18?" Other variations of "tips" have included racial slurs and curse words.



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"Although tipping is an accepted custom in the United States, there is a lot of confusion over when, how much to give, and to whom," Michael Lynn, Ph.D., "tipping expert" and professor at Cornell's School of Hotel Administration, told Yahoo! Shine. "Although no one exactly knows how tipping originated, one theory dates back to the late 1400s in Germany when a silversmith wrote a note to a patron that read, 'My apprentice helped with this and would also appreciate a tip.'"



According to Lynn, the trend took off and tipping became big business in the United States with people tipping more frequently and in larger amounts than in any other country. And there's a psychological component at play. "In addition to compensating someone for providing quality work, tipping can often be about social approval," he says. "We live in a society where status is achieved and not ascribed. Money is a measure of how much we've achieved and often people determine a tip based on how much they want to prove their worth." There's social pressure too. Who hasn't been at a group dinner and sneaked a peek at how much their dinner companion is giving?



But in a modern-day society where people can pretty much get any of their needs delivered (cat food, diapers, toiletries) thanks to the rise of technology, the rules of tipping can be confusing. Do you tip the bellhop for bringing up your bags? What about on a spa certificate you're given as a gift? And do you always have to tip 20 percent—even when service is poor? A waitress could be forgetful and slow but odds are she only makes minimum wage (a paltry $2.13 in many states); does she deserve a sympathy tip?



"The rule of thumb is that you should always tip when someone is bringing you a product or service where manual labor is involved," says Lynn. "For example, movers, food and drink delivery people, restaurant servers, house cleaners, salon workers, and hotel maids. These are also situations where you can measure the immediate value of the work, unlike say, a car mechanic or doctor where their services are more abstract and assessed over time."



Tipping on gifts you received such as flowers and gift certificates fall into gray areas, but in an ideal situation the giver would have prepaid the tip before sending the gift.



There's a tricky tipping scale too. Standard tipping rates are 15-20 percent in many parts of the country but that bracket is rapidly expanding. "People are tipping more these days and the mean tip is closer to 20 percent," says Lynn. "We call it 'tip creep.' One person can boost the norm and then there's upward pressure to conform. Many now tip 25-30 percent."

And what of the pizza delivery guy—why was he given such a poor tip? "Without knowing the specifics, it's possible that the person ordering the pizza just didn't know any better," says Lynn. "Standard tipping on a pizza pie is a few bucks. So he may have figured that delivering 85 pies required one trip—the same trip to deliver just one. But instead of giving him a few dollars, he upped it to $10. The reality? He should have tipped between $85-$100."



More on Yahoo! Shine:

Single Mom, Sorry: Is it Ever OK to Leave an Excuse Instead of a Tip?

Are You Tipping Correctly?

An Insider's Guide to Tipping