I spoke to the Shopping Cart People, and it turns out that they can populate the cart with the old stuff. The downside is that the things I've added will be lost.
But I can put them into a CSV file, save it, let them put all the old stock back up, save that, and then meld the two in Excel, and load all the new stuff in at once. So that's what I decided to do.
Which means, as of this morning, the shopping cart is working again, and you can buy everything. I still can't ship any of it out until the end of next week, but you can buy it. :D