When dealing with a retail establishment (as opposed to a private sale), they should have controls in place to correct such mistakes before the sale is finalized. The key point here is that the error passed through enough people/systems acting as representatives of the seller that I can consider the organization as a whole to approve of the sale.

If a cashier accidentally gives me a $10 instead of a $1, that’s unethical to keep. But in this situation, the error started at the advertising stage and persisted through totaling the order, processing payment, and fulfilling for delivery several hours later, all steps at which, in a traditional business, someone could have flagged the error. At this point, it’s their intentional lack of safeguards rather than unintentional mistake that’s at fault and I have no moral qualms about that.