“It’s similar to a cell phone plan. Depending on your use, if you pay for a bigger plan, it costs you more. If you use Square, there’s no monthly plan cost but the per transaction fee is much higher,” Muir said.

Square can therefore be more compatible with small businesses who do not have many transactions per day.

Several tents at the Harvard Farmers Market now use Square to provide the convenience of cards to customers at their stands.

“People are convinced that you don’t need to carry cash anymore, but that doesn’t work in a farmers’ market,” Brian Quinn, co-owner of Q’s Nuts, said at his stall outside the Science Center Tuesday.

Quinn looked into purchasing a credit card system but did not want to make a long commitment or pay a monthly fee when his business is seasonal.

“All I had to do was get a smartphone,” Quinn said.

—Staff writer Kerry M. Flynn can be reached at kflynn@college.harvard.edu.