[pdurrant]

Originally Posted by Mike L

Ah, yes - especially if you don't stipulate that the recipient has to know that the free gift is a fake.

But wait, in that case .....

Spoiler:

Skip the weighing. Give any ten coins to the person you are buying the product from. At least nine of them will be genuine. The other is the free gift, which might or might not be genuine, but you've satisified your criteria.

And you'll end up with either one or two genuine coins to use another time, which you wouldn't have otherwise. Then, applying the same logic, you would be able to make another purchase which costs one coin.

Hmm ....

I think you've found a flaw in my puzzle.

The next problem for everyone - fix the question to remove the flaw!