Just my opinion, but they should only charge the last payment when they can ship within a couple days time. Is this SS or TW?
That's what they do, that last payment will sit there until the figure ships, if that's what you want to do. I usually just go ahead and pay it off, it makes the whole thing a lot less painful. I had Cody paid off months before he actually shipped.
Some people dislike SS holding their money like that, but to me its just a convenience and I can start thinking about who to pay off next.