So my son comes back with a paper list of the S coins he has available for trade. He's 14.
I circle 2 coins on the paper that I need. He goes away. I put the '70-s in a plastic slide holder to get it ready for him. The next day, I get a coin from him, a '40-S. My other choice was a '54-s. I said that's great, thank you. He proceeds to tell me, "yeah after you circled the coins, I went and checked, and the '54-s is actually rarer, so, I'm giving you the '40-s..."
I give him the '70-s and he says "oh wow! can I keep the slider thing?" My wife and I both replied, "oh, that's an extra charge".