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To refund the customer, go to their Contact Record, click the Purchases tab, then scroll down to the "Transactions" section. Select the transaction and in the Actions menu that drops down, select "Refund Transaction". That will refund the transaction through the original gateway.

Note this will not work with standard PayPal, and you will have to refund the customer through your PayPal account manually. You can then void the transaction in ONTRAPORT.
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