To test with PayPal sandbox, you go to https://www.sandbox.paypal.com/ and register your own sandbox account, then you create a sandbox account for merchant and an account for buyer.

In the PayPal plugin configuration, you enter merchant sandbox account and set environment to "Sandbox".

When you purchase for gift card in front-end, you pay with the buyer sandbox account you have created, if everything goes well you will receive emails.

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Lancaster