I agree with the others. There is nothing wrong with re-using materials if you secure them properly. I have received re-used envelopes where they appear to have resealed the envelope with its original sticky-tape, and it is no surprise one of them was empty. But stapling and taping with packing tape (not regular cellophane tape) is good. I always tape the staples, too, so postal workers don't get cut.

I might have asked for a photo of the materials so I could improve my own quality control. You have done all I could have done in the end, though, by refunding the purchase. There is also a limit to how much time and agony you want to spend on a small transaction.