You should have gotten several codes for digital content with your HumbleBundle. The shipping and handling should not have been 0, it should have been something like 8 dollars, however, potentially did you combine it with some other order or anything like that? If they canceled your order, then they should have reactivated your code. [don't know they did do this, but this might be what you need to ask them to do] If you use a coupon to pay for a 'free' something, and they take your coupon with your order. If they cancel the order, they should give you your coupon back. (since they canceled the transaction you should get back what you gave)
I got my book fine. As I recall it was backordered, and it did take it a while to get to me. However, the others I know who ordered theirs later on, after the backorder was getting dealt with, have all gotten theirs too, one relatively recently.
The back order situation wasn't an issue for me since I already had a first printing book, I got the bundle for the digital content I didn't have yet, and the opportunity to get a second printing with errata in it when they came out.
Unfortunately, it is true Customer Service can certainly be slow because of their own general backlog, but generally the times I've needed to deal with them they have been helpful, and I was eventually happy with things. If you use the Customer Service forum, make sure you don't keep updating your thread asking for status as that just pushes it as a newer thread, meaning it will take longer before one of the staff get to it.
Anyway, I would find your HumbleBundle code you used for your book, you might see if it got reactivated. If not, ask them to either re-activate it or give you a replacement one. If you can find your notification where they say they canceled your order, that should be usable to show you had used that 'gift-card' type entry for the order, and want it back to re-order.
Do note that shipping will not be 0 dollars however, and the HumbleBundle was, as I recall it, pretty explicit about that. But I've gotten some other physical rewards from HumbleBundle in the past and so wasn't surprising at all for me. I think you can add it to another order, or ask for it to get sent with your next subscription, if you have one. That might save you on shipping.
quick edit:
Quick note, don't put your code in a customer service post, as they are visible, and you wouldn't want someone else to see it and if Paizo did activate it again, have someone use it before you got to use it.