I just spoke to a supervisor at BMG customer service. Basically, there was a major fuck-up, and all the box sets were incorrectly marked down to those low, low prices. She said a few box sets have gone out at those prices, and they aren't going to do anything about those, but they've either cancelled or marked "returned" all the rest that they've been able to catch (and they won't be shipped at the extremely low prices).
So that's it folks. It seemed like it was too good to be true, and it was.
EDIT: Just to clarify, the charges for the sets marked "returned" have already been charged to our credit cards, but those sets will not be shipped and the charges will be refunded (hence being treated as "returned").