Sandbox payments are transactions processed while the project is in development mode. The payment processes in a test server, as opposed to your merchant account, so no one is actually charged.
Here's how to process a sandbox/test payment
What if real users make sandbox payments?
It's super important to open your project for registration prior to users registering so their payments are processed correctly, and not through the test server as sandbox payments! If this happens by mistake, follow these steps:
1. Open the project to prevent additional users from doing the same!
2. Issue a manual refund to zero out the sandbox payment
3. Delete the user's payment method, as it's not saved with the merchant account
4. Ask the user to pay again now that the project is open
Note about refunds: Electronic payments are refunded by clicking the 4 dots to the right of the transaction, but sandbox payments can be refunded with manual refunds, since the user wasn't actually charged.