Is there a 'demo environment' that my developers can use?

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Yes there is! You can create a demo account at http://demo.vivapayments.com

This can be used for testing and development purposes and no actual transactions can be performed there. 

Please keep in mind that not all options (such as "Pay Now" buttons) are available in the demo environment.

You should never use actual card data when making test transactions through the demo environment. A successful card sale can be simulated by using the card number 4111111111111111 with any valid expiration date and CVV value 111.

Obvious but definitely worth mentioning, you will not able to use any data produced in the live environment, in your demo account and vice versa!