In US terms, the contract in this case would be to deliver Tearaway. The "improper" fulfilment of that contract was that the delivery was a bundle of Tearaway+Vita. The buyer could then accept that as an adequate substitution, and consider the contract complete, or reject it. In this case, the reason for rejection presumably would be that they didn't feel morally justified in taking advantage of the company's mistake, since it is clearly a more than adequate "substitution" in objective terms.
Elsewhere in the thread, the example was used of a delivery of a medium shirt instead of a large, to show the absurdity of the idea of keeping the medium *and* demanding the originally ordered large, or keeping the medium *and* disputing the credit card charge. Yes, that would be absurd, but it would also be absurd for the shirt company to contact the buyer afterward and *insist* that the medium be returned for exchange for a large, even if it turned out the medium was acceptable.