Sorry for the late reply, but welcome to SaleHoo :)
Honestly its a bit of a surprise to read that you found authentic software keys on DH Gate, especially since we have discouraged our members to buy anything branded on DH gate and other Chinese suppliers.
Since you contacted Norton directly, might have been a good idea to ask about purchasing the keys so you can distribute them yourself without having to buy through DH Gate.
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Hope this helps :)