I wouldn't pay to be honest. You have contacted them and told them about the mistake so there is no dishonesty on your part. My opinion being if I make a mistake I pay for it, if the company/other person makes a mistake they should pay for it.
Why should you be out of pocket for their mistake? That's like being in a restaurant and being brought out a meal you haven't chosen or ordered and told 'well we know it is our mistake but we have cooked it so we will lose money and therefore if you wouldnt mind paying for our mistake we would be very grateful'. You wouldn't do that, priniciple is the same i think.
And by that I mean I don't really want to pay now for something I wouldn't have paid for in the first place. But now I have them if my husband does end up using them then technically he's benefiting from something I haven't paid for...
I bought a brand new pair of sunglasses off eBay from America & found an extra set of lenses in the packaging. I contacted the seller to say it had arrived & what were the extra lenses for & got this reply:
Hi. I think we sent you the extra pair of lens by mistake. I am sure we can't take them back for it will cost more money to return them. You can keep them if you want. But I would appreciate if you at least pay our cost for the lens which is $20. If not, I will be okay. You can keep them anyways.
WWYD? If it makes any difference the sunglasses were Oakleys end of line stock costing £135.