I think it was in Terry Pratchett’s Going Postal that a con man mused that the easiest people to rip off were other con men, or at least those who aspired to be so, because all you had to do was make it look like they were taking advantage of you. Honest men wouldn’t fall for it because they weren’t willing to rip you off.* It’s interesting watching myself fall for that.
A year ago Audible (owned by Amazon) offered me a year’s membership (=12 credits for free books, plus discounts on other books and some miscellaneous perks) for $100. That’s almost half off the per credit price (which itself can be can be half the price of buying a book with money, although those prices are clearly set to encourage subscribing rather than to be paid). I was about to start reading a bunch of dense history tomes, so this seemed like a pretty good deal. Then between my plan changing and the library gods being generous, I ended up not using a single credit. Suddenly 11 months had passed and Audible e-mailed me telling me they would renew my membership in a month.
“NBD, I’ll just cancel my account and keep the credits for when I need them” I thought quietly to myself. Only it turns out unused credits disappear if you cancel your membership. To keep them, I’d have to renew my subscription. But Audible only lets you keep 18 credits at a time. If I renewed for another year, 6 credits will immediately expire, meaning I’m paying for 12 credits but only getting 6. So I went on a spree buying books off my Goodreads queue and gave a few as gifts. Actually I gave one too many gifts, because I realized later there was another good candidate I didn’t have the credits for.
Audible to the rescue. No sooner had I spent my last credit than they offered me three for $30 each- more per credit than I’d paid for my first membership, but less than the annual membership. I went as far as picking out two more books before realizing this was stupid, I had gone an entire year without buying a single audio book, I did not need three more credits.
Then my roommate told me $10/credit isn’t even that good a price, they were sure to offer me a better one when I actually cancelled. And I wanted it. A product I empirically had to be forced to use, and I felt compelled to buy more because it was cheap.
I put this down to two things- “Audible subscriber” was a nice identity to have and I enjoyed the feeling of pulling one over on Audible. I was the dishonest man letting the con man fool him. And I knew this was happening and it was still an act of will that I actually cancelled my subscription.
This story has a happy ending- well, except for the part where I paid $100 for a bunch of books I only marginally wanted. I probably still captured some surplus. But this can not possibly be the only trick Amazon is playing on me, and I don’t know what to do about it.
*I tried to find an exact quote for this and instead found “There is a saying ‘You can’t fool an honest man’ which is much quoted by people who make a profitable living by fooling honest men.” But I’m pretty sure the part I quoted was there too.
Aceso Under Glass 
David W Maurer’s The Big Con reports that this is in fact widely believed by con men:
This is because cons generally involve making the mark complicit in getting one over on someone else, since they’ll overlook irregularities if they think someone else is the sucker. For much more detail on how this works with many concrete examples, read the book.
Related: Out to Get You
There’s a song which describes the a con-a-con situation: https://youtu.be/hFcipARBJb8
These “free book” scams have been around for decades. If you have to spend money for something you didn’t want just because it seems like a good deal, then it is not a good deal!
I think the saying you are looking for is “a fool and his money are soon parted”.
Just use Kobo. They cost more, but you don’t feel like you need to take a shower after interacting with them.