I mean let's be clear here, they had a bunch of artwork laying around already (we know this), artwork that ended up in a wallpaper, and they had rewards of $30 - $50 for the game + some digital stuff (e.g. an mp3 of the soundtrack) + getting this artwork mailed to them. Oh and the game was just $10.
So in effect they have a PDF on their computer, and they're charging $20 print and send it to you. (this isn't even the booklet, just the wallpaper, the booklet+wallpaper costs $40 extra ontop of the game).
Now you find me a scrappy indie game dev that doesn't want that, or can't handle it, or can't make that profitable. Don't act like that is rocket science. You telling me if you're running a small shop of 5 people and you're so scrappy you'll risk bankruptcy 6 months later, that printing and shipping a PDF for $20 to 1000 people isn't workable? That generating this $20k revenue (1/3rd of what they pulled in) from a wallpaper, can't be done for less than $10k everything included (printing, labor, shipping).
I mean hell I can print 1.000 A3s for less than $1k without much effort, already found a quote for that, and it'd be printed within a day. Again, they said they had everything printed and ready, that's not the issue here. Then I reward myself $3k for an entire month of effort of shipping pieces of paper (big deal, could do it in a week), and I spend $3 to mail 1 piece of paper each. I still make $13k, enough to pay a developer in Belgium for 3 months. And considering I spent $3k on the labor, I might as well outsource it to some highschool intern who gets a ridiculously royal salary of $3k for barely a week's worth of work. And I'm being pretty generous with the margins here, even if you nitpick here and there, and add some other factors, it's still a good deal. In fact I doubt they regretted their decision, merchandise was half their campaign revenue.
Now I KNOW there are companies that offer ridiculous stuff. 1) offer things they haven't made yet (like a mini comic to go along with the game, which requires a lot of time spent) and 2) things that make no economic sense (a $20 reward for a 3d printed game thingy, which costs $15 to make and $10 to ship, and time spent). And that this makes no economic sense and often fails.
My point is that while merchandise isn't always a good idea, it definitely can be, and that it probably was for this company.
As for a 'book order', it's just a booklet made out of a bit of paper, at a quantity of 1000 of them they cost about $1 each or so. Remember they're part of a $40 reward which has a booklet, wallpaper (combined <$2) and digital rewards like a soundtrack. Hardly unfeasible, this is on top of the $10 of the game's price.