You are getting into "he said - she said" territroy there. t's coming back, just bite the bullet and get it over with.
You may have protected yourself by stating that defectives would be replaced.
Such new items are referred to as Dead On Arrival. Unless you open and test each one, this will happen with electronics. You need to work within the manufacturer's warranty policies to have something that is fair to your customer, but still keeps you covered. In most cases, DOAs are replaced at no cost to the buyer, including shipping both ways.
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