Another thing is I don't think you've really had a lot of experience with commissions because reading over the whole post, it looks like you've started already(?), even though it seemed like OP didn't seem 100% set on choosing you.
Doing a book cover takes planning, especially if it's a paid job, so you want to get as many details as possible, maybe make up a couple mockups or first iterations and make sure that the other person is satisfied with it. And before anything, I send an invoice in case clients ever change their minds halfway through.
I don't know if you've sent OP some examples that you've done separately, but you'll see a lot of people here linking a DA, their portfolios, or just examples of their work so anyone interested can know if the art style is what they're looking for. These are just a few tips, most of this is learned through experience.
edit: grammer