It's something that owner Mason Wartman said he started doing about two months ago, after a customer mentioned an Italian tradition that allows people to pay for coffee that the less fortunate can redeem later.
The program "honestly helps a lot of people," said Wartman, 26, of Center City, who majored in business at Babson College and worked on Wall Street, where he helped write reports about stocks.
"A little bit of effort that we invest on our end makes a world of difference to a lot of homeless people," he said.
He said the shop's pay-it-forward model is one of the most "efficient" ways to do charity because the slices are paid for in advance - which increases sales - and the restaurant has no additional overhead costs.
Inside Rosa's, on 11th Street near Chestnut, hundreds of yellow, pink and light-blue sticky notes with phrases such as "God bless you" and "Eat Up & Don't Give Up" line a wall. They were written and left by customers who received a sticky note after making $1 donations for a slice of pizza.
The sticky notes used to be how Wartman tracked donations. But nowadays, he said, he uses the register to record donations, which exceeded 825 at last count. The notes remain more a symbol of encouragement for the homeless, who can simply walk up to the counter and ask for pizza.
Thomas Netzel, 27, of Mayfair, donated $3 to the program on a recent Wednesday.
"It's paying it forward," he said, shortly after taping to the wall three yellow notes that read "Enjoy."
Ryan Galloway, 29 and homeless, agreed.
"It's an awesome concept that he's doing," he said. "Homeless people are always hungry, and it's a chance for people who have [money] to give back."
For Brandon Koldus, 30, the free slices have been a break from shelter food, which he said "tastes like crap." He started coming to the shop when it opened last December because of its $1 slices.
Koldus, who has a thick brown beard, green and blue ear gauges and tattoos up and down his arms, said that when you're homeless, "people don't look at you like you're even human" and seldom help.
"It's nice to have somebody that's actually got a heart and understands that times get tough," he said.
To donate money for slices of pizza to feed the homeless, go to squareup.com/market/rosas-