Your money is either in charge of you or you’re in charge of it, there’s no middle ground. Using some type of personal finance software can help alleviate some of that money stress and better allow you to manage your money effectively. Without it, you may just be setting yourself up for constant financial worry. Life is already tough enough and there’s no need to make it more difficult by simply hoping your money issues will all work out in your favor. Hint: they won’t.
You can sell your ebooks through Amazon's Kindle program or Apple's iTunes Connect, which gives you access to a large majority of the digital-book reading market and the barriers for entry are incredibly low. You don't need to invest lots of money to make this happen, but you do need to invest lots of time, not only in writing the ebooks, but in marketing them as well.
As I get older (and hopefully wiser), I’ve begun to understand some of the money mistakes I’ve made along the way. After reading Why Smart People Make Big Money Mistakes, I see that I’ve fallen prey to many of the issues raised by this excellent book. This book does a wonderful job of explaining how humans are naturally inclined to focus on the short term and have multiple inherent biases in their reasoning that lead to big money mistakes. More importantly, it provides thoughtful approaches to minimize those mistakes so that the reader will be better able to manage their finances.
You'll also need ecommerce software, fulfillment software, worry about warehousing, customer service, refunds and so on. But that's not all. You'll also need traffic. Think search engine optimization, Facebook ads, and other social media campaigns. Sound like a lot of work? Sure, it is. Especially if you do it all on your own. You could opt for Amazon's platform, which might be the easier route. But, then again, at the end of the day, this is a serious business, which could produce significant profits. So you're either all in or you're not. 
What’s the catch? None, really. Cash back apps act as affiliates for many online merchants, which means that whenever you make a purchase through one of the apps, they get a small commission — but then, they give you a portion of that commission as “cash back”. For example, if I buy a pair of Nike shoes through the Ebates app (or website) and spend $75, Ebates may get a $10 commission but then they’ll pass $7 back to me. It’s basically a way to get sale prices on stuff that isn’t on sale!