In order to donate a gift certificate, you would first want to add the gift certificate to the ticket as if you were going to sell it, but do not close the ticket yet.
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Once you have added the gift certificate to the ticket, simply double click in the discount column for the row with a description of "Purchased gift cert #". Within the discount column, type 100% and press Enter. If there is nothing else on the ticket, you can then press Check Out/Close. If there are other items on the ticket, you would want to process the payment and close the ticket like you normally would.