While working in test mode, you can easily initiate a dispute on any captured charge. By going through the steps you'll get familiar with the dispute process and learn all the possible outcomes.

Start by signing in to your dashboard and opening any captured charge. You’ll then have access to the following button:

Continue by typing in your dispute message and clicking Submit dispute message. You can then choose either one of the following options as the outcome for your dispute.

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Disputes
How do I manage a dispute?