While I don't like Ed's tone, I have to admit ther ereally wasn't much need for discussion. You a, get ahold of em and explain what happened > than offer to return the difference with the package. If he doesn't want the difference then hey feel free to keep it.
Like I paid for a computer few weeks back, ended up sending 5 more than was needed. Guy e-mailed me stating such, I said don't worry about it (he had over estimated shipping)