Is it ethical..?

I've gone for both options in the past. If I'm in Tescos and they give me change for a twenty when I gave them a ten, I'll keep schtumm. If it was my local corner shop, I wouldn't.

Similarly, if I'd been sent two expensive doohickies from Wiggle, I'd probably STFU about it, but if it was someone like Charlie the Bikemonger, of course I'd send one back.

Does that make me bad?

Having been a checkout girl, this actually happened to me, except it was a scam, and I got in trouble. The checkout person isn't Tesco, they're just somebody working there who is liable for your not returning the money. Eh, just trying to point that out.