I'm glad more people do this. I did the same thing once, put a coin in my pocket (it was a tiny thing) and by the time i got home it was nowhere to be found. I felt horrible and emailed the owner.
They were completely fine with it and were extremely understanding. However I just couldn't let it go, so I finally got them to agree to accept one of my unactivated coins. I sent the tracking number to them, got them to activate it and they told me just to place it in a cache with hopes it would make it back to them. unfortunately the first person to grab it kept it. the irony of this, if not so annoying to me may actually be funny.
talk with the owners. sometimes an exact replacement is great but if none can be found a different replacement coin from an unactivated collection can be just as good.