I was originally going to put:
"Just dropping this off before I keep forgetting about it. It being a paid piece, it's something that should've been corrected immediatedly, and for that apologize.
But I feel that's too "cold". I'm terrible with words, so I'm hoping someone here would be able to help me with this? Thank you.
Edit 1/26/16: I was able to give him his commission. And at first all things seemed to have gone smoothley, but several days later he came back to me, asking to talk because he claims he's tired of "running" and feels there's unfinished business between us. Now, he's not being malicious in his words, he just feels there's something to patch between us, but I don't want to, nor do I feel anything needs to be "patched". I just wanted to give him his commission and be done with it all. But I can't if he won't leave it alone. I'm not asking for advice for this part, because I already responded to him, letting him know that I personally would've been fine to have just given his commission and be on my way, but I decided to let him talk. Stupid move, I know, but I've already said it, and I would feel bad taking it back. So I suppose this is now on me. Still though, thanks to everyone who gave me advice.
Edit 1/31/16: Okay. I was more firm with him this time, and he backed off without any fuss. Things are settled between the both of us, and it looks like things will and should be smooth from here on out. Thanks once more to everyone who has helped me with my original issue.