A gift is defined as an item you are sending to a friend or relative personally and should include a card or other notice indicating that it is a gift.
As per U.S. Customs the maximum value for gifts, donations and documents entering the United States tax and duty free is $100 USD.
To help facilitate quick transit through customs checkpoints a record of the purchase or transaction, such as an original sales receipt should be included with the shipment. This is to confirm the value of the shipment in the absence of an invoice.
It is best to not gift-wrap these items as with all shipments which cross borders, they are subject to inspection by Customs officials. For more information, please refer to U.S. CBP's guide to sending gifts to the U.S.