That wasn't a glitch in negotiating. The system knew that the purchase was cancelled but sent the items anyway. It makes perfect sense that the system can legally ask to get those items back.
It depends on what the cancellation policy is, if the cancellation policy says that the responsibility to return is on the buyer, then absolutely it's stealing.
90% discounts are not unheard of.
can you not think of corner cases to your vague notion of "obviousness"?