Let's talk ISBN numbers. Do you purchase them as you need them?
I don't want to spend $22 every time I need one, especially since I would ultimately need 4 for each book (ebook, audible, paperback, hardback) when for a little over $500, you can get 100 ISBNs.
I know that is a lot up front, but you will have the numbers forever and don't have to worry about purchasing them for every project.