At that weight, she'd have been better off sending it in a 3kg prepaid satchel, unless its size prevented it from fitting. Annoyingly, I've had to buy 5kg satchels for items weighing just over 1kg because they wouldn't fit in a 3kg satchel but it was still cheaper than sending them via parcel post with cubing.

Anyway, good on you for having the decency to cover her loss by sending extra money as a personal payment, because most people wouldn't bother. I'm much the same as you, and couldn't sleep at night if somebody made such a terrible blunder and ended up losing money on the transaction. Sure, it might be her fault for being careless but that doesn't give us the right to profit from her mistake, and besides, karma has a funny habit of catching up with those people who do.