Leanpub has three plans for authors: Free, Standard, and Pro. Here is an article with a description of the different plans.
You can see your current subscription here:
If you are on a paid plan, you can see your past subscription payments, and get receipts for them, here:
Please note that Leanpub authors can also pay for Shelf spots and "Sponsored Deals" positioning in our email newsletter sales, in order to help promote their books., and they can actually use unpaid royalties to pay for these promotions, so their books can potentially become self-financing.