I don't see any moral issue until Amazon indicates it made a mistake and is actually asking for the return of the goods or for more money. I haven't seen them do that. Maybe their intention was to provide a great deal to their customers.

Even if it was unintentional the relative cost to Amazon is tiny and but the relative cost to any individual customer could be quite large. What you can afford to pay is not absolute so damage caused is not equivalent between an individual and a billion dollar corporation.

In life and money there are no moral absolutes and treating all corporations and individuals the same is not necessary. For me, immoral actions are those with the potential to cause harm, and I don't see equivalent harm in your two examples.