If a friend bills you $4 for a favor, are they really a friend?

When Jonathan Arnold felt sudden, severe abdominal pain, he contacted a friend, who rented a Zipcar and drove him to the hospital, less than 10 minutes away.

Arnold ended up having emergency stomach surgery. A few days later, he also got a bill: a $4 Venmo request from his friend for the hospital drive.

He was upset, not least because the friend’s finances were better than his. “Needless to say, we are no longer friends," said Arnold, now 23.

Payment apps that allow us to send money seamlessly are turning us into each other’s accountants, itemizing charges from a $3 coffee to a $60 theater ticket. Tools like Venmo, PayPal and Splitwise make it easy to ask friends to chip in for a pizza or an outing—and pressures the one who always promises to pay you back to actually do so—but few things can sour a friendship faster than an unexpected bill.

“It’s a shock when you get the PayPal request, and you’re like, ‘What? How much was that?’" said Myka Meier, founder of Beaumont Etiquette, a New York-based etiquette consulting firm. “That can be awkward for both people."

Friendships, no matter how old or treasured, always contain some degree of scorekeeping and payback. After covering the check at dinner or hosting people for a cookout, it’s reasonable to expect guests will reciprocate.

The impersonal nature of after-the-fact Venmos, especially when the person seeking repayment never mentioned they would send a bill, have people rethinking what’s a friendly gesture, and what’s a billable cost.

Payback time

Venmo, Splitwise and other apps let users nudge each other with reminders of outstanding bills. Venmo’s transactions tab, PayPal’s and Cash App’s activity menus and Splitwise’s home screen keep users’ debts visible for months or more, making it hard to forget when someone owes you money, or when you owe them.

In 2022, Matt Mullin covered his friend’s $65 round of golf. He sent a Venmo request for the bill immediately. No payment. He sent a reminder six months later. About two years after that outing, the 31-year-old from Chicago is still waiting.

“Every time I log in, it’s right in my face," Mullin said. The debt has become a running joke in their friendship.

The debtor, Joe Jasnowski, 31, said he is paying Mullin back the old-fashioned way, springing for drinks and Ubers while they’re out.

“Maybe I’m still $30 shy, but it will all be paid," Jasnowski said.

Most people pay their debts promptly, said PayPal, which owns Venmo. This year through June, more than 78% of users who received a money request fulfilled it on the same day, while 17% paid within five days. Some 4.5% took longer, or allowed requests to expire. PayPal requests expire after 60 days; Venmo’s don’t expire.

Payment apps have been around for years. PayPal dates back to the dot-com boom, making it as old as a Gen Z user. There are 90 million U.S. Venmo users, and Pew Research survey data showed 57% of 18- to 29-year-olds reported using it. The code of conduct is still being worked out.

Petty cash

Asking for small amounts of money can make us feel small. So many of us let it slide when, say, we pay for parking (and gas) when driving friends to a concert, chalking it up to the marginal cost of having friends.

Technology has removed that barrier, making it easier to nickel-and-dime friends and acquaintances. One point of debate among users is when a request seems reasonable, and when it seems miserly.

Social media is full of people irked over trivial Venmos. A Reddit post last year fuming about a weekend guest who sent her host a request for $1 for a muffin, among other small items, generated about 1,800 comments. Many commenters said it was cheap, passive-aggressive behavior.

Payment requests of $5 or less are considered rude by 72% of people ages 16 to 26, according to an April survey by the payments service Cash App.

The treats that aren’t

And then there are the diners who magnanimously order something “for the table" without telling their dining companions they’re on the hook.

Quianna Dinkins, 30, a project manager at a tech firm in Chicago, attended a New Year’s Eve brunch with friends where an acquaintance ordered a round of tequila shots for the table. She promptly paid her share of the bill, $73, but got a Venmo request two weeks later asking for $30.

“It was a random notification, and I wondered, ‘Who is this person?’" Dinkins said. “Then I saw the memo said ‘brunch.’"

It was the tequila shots, which her companion never mentioned weren’t gratis. Dinkins said she found the request unfair but still fulfilled it.

She also would have liked to put her card down for the whole meal, to get the credit-card points and collect money from the others, but the acquaintance beat her to it.

“We don’t have hard rules yet for when you’re supposed to send a payment request or how long you have to respond," said Lindsay Bryan-Podvin, a therapist who specializes in financial behavior.

Communication breakdown

It isn’t just the money; it’s the surprise that makes us recoil.

People often avoid uncomfortable financial topics to sidestep feelings of shame. So rather than discussing costs openly, people defer to digital requests—even though that can backfire when the recipient is blindsided, said Meier, the etiquette expert. She advises talking it through.

“Be upfront early: ‘Here’s what we’re buying, and here’s how we’re splitting it,’" Meier said.

Sending payment requests promptly is another way to avoid pitfalls.

Justin Wolfskehl, 26, went on a ski trip in January with more than a dozen people. During a night out at a busy bar, the group ordered drinks for each other whenever they managed to get the bartender’s attention. One man nabbed some drinks for Wolfskehl.

“At first, I thought, gee, what a nice guy," said Wolfskehl, who lives in Vail, Colo., and works in accounting. “I’ll get him drinks the next night." He said he did so.

To Wolfskehl’s surprise, the man, a friend, sent him a $35 Venmo request in April, months later. Not wanting to create tension in his social circle, he paid. Unhappily.

“I looked at the request for like an hour, thinking I’m not going to pay this," Wolfskehl said. “I finally sent it, but didn’t say anything else to him."

Write to Dalvin Brown at dalvin.brown@wsj.com