How to Use Apple Pay Cash
Remember that if you pay with a credit card, there’s a 3 percent credit card fee, which I learned the hard way.
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Under Settings > Wallet & Apple Pay, you’ll find a new Apple Pay Cash card—tapping on that gives you a few additional options, including a transaction history, whether or not you want to automatically accept payments, and ways to add money to your Apple Pay Cash balance or transfer your balance to a bank account. Remember that when you have a balance on your Apple Pay Cash card, you’ll be able to use it like any other card you have in Apple Pay. (I’m not sure yet what happens if there’s not enough balance to cover your purchase—does it simply fail or fall back to another card?)
Apple’s got a more thorough help document on Apple Pay Cash if you’re curious, as well as one that details the monetary limits.
Update (2021-08-06): Francisco Tolmasky:
Oh hey look at that, Apple decided to turn the money knob.