I always do the honest thing and at least contact the retailer if they make a mistake. Bodybuilding.com sent me four times the product that I ordered. They were awesome and said keep it. A large gunshop in Columbus, OH forgot to charge me for five boxes of ammo and neither I or the cashier caught it. I realized on my way home that there had been a mistake and called. The woman on the phone acted like I stole the ammo. I said I would give my credit card over the phone and she acted like that was not possible and repeatedly told me I had to come back in. I told her that I wasn't going to do that, as I was already two hours north. I gave her my phone number and told her to have her manager call me if he wanted the items paid for. He called later and it got taken care of. Some people in customer service make it hard to want to be honest, but I still try.

If they told you to keep it, keep it.

Primary Arms double ships stuff to me on occasion. Once I verify that I have not double paid, I let them know and they send me a shipping slip to send it back.

Anything less is theft.

Were it not for the cost of shipping I'd return it. And since they said to keep it, I think there's no dilemma.