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The key difference between the two "complete refund" buttons is how the order will end up being treated in your period sales reportPeriod Sales Report. Orders that are rejected, while remaining in the order list, are not included in the sales reporting. So, if the customer contacts you immediately after the placement of the order (before its packaged and shipped) you can use the complete refund Complete Refund (then reject) button to effectively void the sale.
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