Light wallets are not a problem for Joe Mangrum, a sidewalk sand painter in New York, who has been using Square to take donations from passers-by and sell copies of his book. Sales have increased sharply since he started accepting credit cards on his iPhone, he said. “I’ve made the sale as opposed to twiddling my thumbs because they don’t have the cash.”

The new services could have the biggest impact on the smallest businesses, like farm stands or house cleaners, that accept only cash and checks because they do not have stores to house credit card terminals and do not want to enter into complicated, long-term relationships with credit card companies.

Rachel Ancliffe, a clothing designer in Portland, Ore., sells her dresses and blouses at sample sales and from her home, and uses Intuit GoPayment to process credit card payments.

“You can’t accept checks because then you’re just kissing your product goodbye” because of fraud and bounced checks, Ms. Ancliffe said. “I sell 10 times more because I take credit cards.”

Fraud protection offered by the credit card companies is the same as when the card is used at a cash register. Some of the new companies say security against fraud might even be improved because they provide e-mail receipts, and those from Square include photos and a map of where the transactions were made.

Image Brian Kusler and Nina Ramos, in San Francisco, using phones to exchange money. Credit... Jim Wilson/The New York Times

The death of cash has been predicted since the 1970s, when electronic transactions like direct deposit of checks were introduced. But most digital payment experiments, like one in 2006 by Visa, have focused on swiping cellphones, as is popular in countries like Japan, instead of credit cards. Mobile payments have not taken off in the United States because Americans are just as happy to reach into their pockets for a plastic card as for a cellphone.