SandBox from PayPal is a testing environment for managing payments without being charged. For a proper workflow, you will need a Merchant sandbox Account and a Buyer Sandbox Account.
It’s recommended that you run a few tests using the live PayPal environment, but with insignificant prices (ex: $0.01) for a foolproof simulation. If you’re still experiencing issues, please drop us a quick line and we’ll see how we can help.