Odds are that they will charge you for shipping (from your card on file, like they have done to people who didn't pay enough for shipping), or they will forget to send it... and you'll spend the next couple months convincing them that they promised to send it.

I know it is not your fault, but I'm amazed at how easily they were willing to send out a replacement (they shouldn't have too many extras yet, as the first two are the ones everyone pays for... it's the 2nd and 3rd pairs that drop off in numbers as less people pay the later instalments), and not get you to post back the other one.
I guess on the balance of it, it would be cheaper for them to send out a second one (I assume you had to send them photo proof), than paying for the additional postage of the faulty item back to them.

(hehehe... if they had asked for you to post it back and they were going to pay to you to do it, I would have seriously considered over-charging them about 50% like they do to us, and say that you were using their International Postage policy )