I feel like there's something missing here, did the guest leave early without advising you first? Or did you not make checkout arrangements with each other?
I once had a host show up late for my checkout, and I couldn't just leave because I had the only key to her place and it would have been bad form to leave the key in the apartment with the door unlocked (she was a new host and had a lot to learn, it wasn't the first time I had to take care of her instead of the other way around). Fortunately, I already knew lateness was her norm and planned for it, so I was irritated but it didn't impact my travel arrangements. Anyhow, maybe your guest was acting in bad form for the checkout, I don't have enough details to know, but I can imagine it would make sense, at least it would to me, that your place would be secured and you would have a key to your own post box to retrieve the door key. Personally, I think it would be in bad form to charge the guest for the locksmith.