I feel like some people, perhaps intentionally, perhaps not, are being rather judgemental. If you're in a store, sure let the cashier know you underpaid. When it's online, it can be a hassle to return things, and I'm guessing 99% of the time they will say just keep it. Like, you can tell them, but please don't act like it's super immoral to not. There are far worse things people do that are immoral than accepting something that was accidentally sent to you. I'm guessing the op didn't expect some people to respond in what I consider s rude tone. Like. Give your opinion, but don't act like you have better morals. Morals and values are relative and vary by person. Sorry if I'm sounding kind of harsh, but I really dislike people attacking someone in the name of "morals" when all they did was ask for an opinion.