Are you getting affiliate fees on your royalties?

If you are selling books at Amazon you are probably getting a 35% or 70% royalty on your sales. If you sell those books at Smashwords your royalty will probably be 70.5%. You could be getting 39 - 43.5% or 74 - 78.5% at Amazon though and vastly increased sales at Smashwords. The trick is to take advantage of affiliate payments which can offer you anything from 4% to 70% of the price of an e-book.

Let's deal with the better known (but less profitable) one first:

Amazon 'associates' (affiliates)

You must have a web presence acceptable to Amazon to be accepted as an affiliate. By a 'web presence' that means a website or blog with substantial traffic. You'll probably need to register for affiliate accounts at .com, .co.uk, .ca etc. Australia doesn't seem to offer affiliate services. This means you'll get multiple affiliate tags making setting up links tricky (but not impossible). Amazon's affiliate agreement is a model of obscurity and makes it difficult to understand if a .com affiliate link used by a .co.uk customer will earn an affiliate payment. You can get affiliate payments for virtually all Amazon products other than free products. Amazon depreciate adding affiliate tags to free e-books and warn that doing so may cost you all your affiliate fees if you exceed 80% of links being to free products AND 20,000 free products are ordered using one of your links - unlikely for most people. You won't earn anything on your links to free products but if a customer goes on to view and purchase other items at Amazon, you'll get affiliate payments for those.
Fees are listed at https://affiliate-program.amazon.com/gp/associates/help/operating/advertisingfees
At a minimum, assuming you make less than six affiliate sales per month you'll get 4% for e-books and other digital products
$0.99 - 4¢
$2.99 - 12¢
$3.99 - 16¢
Fees rise to up to 8.5% for 3,131 affiliate sales per month. If you managed to sell 3,131 99¢ ebooks in a month you would earn $263.47 from those sales.
Payments are made 60 days after the end of each month and can be by direct bank deposit, check or gift certificate. Affiliate payments are taken from Amazon't share of the purchase price, not the author's.
Amazon provide a toolbar for affiliates which allows you to quickly generate links and promote to social media.
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Smashwords affiliate fees

Smashwords offers affiliate payments of 11% to 80.5% of the retail price of e-books. The actual percentage offered is determined by the author. The default is 11%. Anyone age 18+ with a Smashwords account is eligable to enroll.
Smashwords encourages the use of affiliate tags on free e-books and the author's own e-book links. You won't earn anything on those e-books but if a customer goes on to view and purchase other items at Smashwords, you'll get affiliate payments for those.
Affiliate fees come from the author's royalties. The author can choose not to offer affiliate payments. If an author elects to not offer an affiliate program for a book then the author's royalty is 85%.
Affiliate links are easy to create. My preferred method is to append the ?ref=[yourScreenName] code to links where [yourScreenName] is the bit after https://www.smashwords.com/profile/view/ on your 'My Smashwords page' at https://www.smashwords.com/profile
Assuming you make those 3,131 affiliate sales of an 11% 99¢ ebook you would earn $340.97 at Smashwords.
Smashwords affiliate agreement is much easier to understand than Amazon's Check it out at https://www.smashwords.com/about/smashwords_affiliate_documentation

Want to earn at Smashwords? All my e-books there offer a 35% royalty. They are visible at https://www.smashwords.com/profile/view/JChapman/authors#published?ref=JChapman If you want to use that link then just replace the 'JChapman' at the end with your screen name. If you manage to sell Amazon's 3,131 books we'll both earn  a minimum of $3,298.51