I strong arm it in photoshop.
I pick the best sun or moon shot and use that as my base shot. I open the base shot in photoshop, and then I bring in all the others in the series as new layers.
Then I just lasso select around the object (ISS or Mercury), select inverse and hit delete. That leaves just the little object on top. I do that for all of them except the base and there ya go. It was super easy for the ISS on the sun, since I only got 4 frames with the ISS crossing the sun in 0.9 seconds. I suspect your lunar ISS will similarly be easy. How did you get 13 frames though? With Mercury on the other hand I had the bright idea to take a picture every 10 minutes. So I had about 33 images to cut out. Also, since it was drawn out over 5 hours, I had alignment issues, frame to frame, that needed minor adjustments in Photoshop on each layer to be right on top of the base. And on top of that the changing atmospheric conditions caused a lot of exposure variations that needed to be corrected. It took a little longer:)
I hope someone laughs at this and tells us about some free software out there:)