Take again my 17x17x17
for example. Even though I put in big letters that people should NOT click the "Add to card" (which is just a big dummy cube), almost all my customers did actually click that button, and only through the manual intervention of Shapeways they got their puzzle instead of the dummy cube.
Interesting... this does pretty much what I thought it did. As for using in it in the Shapeways Shop, I have an idea...
Take your 17x17x17
as the example again. Could you have the dummy cube listed in your shop with the "Display to the public" box checked and NOT check the "Offer for sale to others" box? If you did this I believe it would removed the "Add to Cart" button on the right but it MAY still leave the "Add Set to Cart" button. Could you try this? I'm currently having to work though the weekend and I'm not sure when I'll have some time to play with this new feature.
Even if that does work there are still 2 problems with this "solution"
(1) The model would state "Not for Sale" on your main shop page. You wouldn't know you could order the set until after you had opened the page for that model and saw the "Add Set to Cart" button.
(2) You've lost the ability to specify anything but the default printing material.
P.S. Dare I ask just how many 17x17x17's you'd sold? The "almost all my customers" comment has me thinking you've had at least 3 people buy this. If there were just 1, the statement would be "all my customers". If it were 2 and only 1 made this error, then I wouldn't call half my customers "almost all". 2 out of 3 I would think is the minimum requirement for such a statement. If so I'm very impressed. Congrats!