Is Zeffy really free?

For the nonprofit — yes. Zeffy charges no platform fee and no processing fee, and states that "100% of all payments you receive through Zeffy go directly to your nonprofit." It is funded by an optional contribution ("tip") donors are asked to add at checkout, on top of their donation. Zeffy says "on average, 2 of 3 donors leave us a tip."

What "free" means here, precisely

All of the following are Zeffy's own published statements (sources and dates on the methodology page):

Worked example

Illustration of the tip mechanics. The 10% tip is an assumption for illustration — Zeffy's suggested amount varies by transaction and is not published as a fixed rate.
ScenarioDonor is chargedNonprofit receivesZeffy receives
$100 donation, donor declines the tip$100.00$100.00$0.00
$100 donation, donor tips 10%$110.00$100.00$10.00
$1,000 donation, donor tips 5%$1,050.00$1,000.00$50.00

The pattern: the tip never comes out of the donation. It changes what the donor pays, not what you receive. Whether your donors are comfortable being asked is a judgment call only you can make for your donor base.

What to verify for your own case

See the donor-side cost on your numbers

Zeffy next to the do-it-yourself PayPal baseline. The tip slider sets your assumption for the average donor tip:

Full ranked comparison across Zeffy, Givebutter, Donorbox, GoFundMe, and PayPal: the calculator.