Classic payment
A normal payment. If you accept, the donation is added to the same transaction and you sign once.
Demo
Try Spare on Stellar
Connect a wallet, make a small demo purchase and see the donation step before you sign.
This is a demo. The merchant and the foundation are demo accounts, not a real shop or a real charity.
Using a Ledger? Confirm each transaction on the device.
Soroban call
A USDC transfer through a smart contract call.
A contract call can hold only one operation, so the donation is a separate payment sent after it succeeds. It needs a second signature and it is not atomic: the first transaction stays even if you skip or fail the donation.
Privacy: the memo spare:spare-demo is public on-chain. It names the app, never you.