Screwed myself over
I just wanted to buy some books and didn't care about a membership. I bought one regular membership and 5 books ($5 + $7 * 5 books = $40).
Since a premium membership comes with a credit good for one book, the cost is $11 + $6 * 4 books = $35. Since I didn't realize this, I am screwed out of $5.
It turns out that a premium membership is always better than a regular if you want at least one book, and it's better than Amazon if you want at least six (not even counting the lack of bonus material on Amazon).
If I ever want to buy another book, I should logically upgrade to premium for $7 (assuming that's possible, it isn't clear if I can do this). If I had gotten premium from the start my next book would be $6, so I'm actually screwed out of another dollar too. If it's not possible to upgrade until the next month, I should end my subscription and wait until I want to buy at least six books, then buy a month of premium, rather than trying to buy them now. (now: $7 * 6=42, with premium later: $11 + $6 * 5=41).
I have no idea what to do about this. I can afford $
65 but I am very annoyed at the buying process being so arcane that it's easy to mess up like this.
Upgrading to premium takes effect immediately (or at least it did when I did it), and they will credit a pro-rated amount of your regular membership purchase towards it, so you won't be out any money (except for the amount of discount you got on credits, which is already spent)
The loss of the discount on credits is the main point. I'm out by $5
or $6 (if I ever intend to buy any more books)because of the consumer unfriendly buying process.
Edit: My math was off. Upgrading to premium should be $6, not $7, so I don't lose an extra dollar, just the initial $5.