I think you did just fine. The customer service people were probably surprised to have someone bring back money they were given by mistake, which is a sad commentary on today's world.
About a week ago Mr. Sirius and I took our cats to the vet, and while we were there we purchased some Advantage flea treatment for them. We were supposed to have gotten 5 tubes; we were given 6. The next morning I called the vet's office to tell them about it. I had no problem with paying for the extra tube; we'd certainly use it, and it would save us having to go to the office to buy another one. The receptionist told me I didn't have to pay for it, and thanked me for being so honest.
Barnes and Noble once included a few books I hadn't ordered with my order. I called customer service and offered to return the books if they'd send me a shipping label. The CSR was honestly surprised and very thankful that I wanted to return the books. I figured I hadn't paid for them, so they weren't mine to keep.
Besides, the books were rather..um....well. let's just say the paperbacks were romance novels of the more steamy variety, and the big hardcover was a very detailed "how to" book on playing scrabble, complete with rather explicit illustrations. Not exactly the kind of thing I keep on my bookcase, y'know?