After two weeks of phone calls and e-mails, I finally got an e-mail back this morning from the executive escalations department at PayPal.
Here's the money shot:
regarding the system is not passing the item number or the title through with the address and buyer's information on multiple shipping orders, we did not have a known issue registered for this concern or a solution, so we submitted this issue to our technical department so they can assist. Unfortunately they do not have a timeframe for when this will be resolved.
Translation: we don't know why this is happening, we're not going to help you troubleshoot it, and you're fucked. Thanks for your tens of thousands of dollars of business over the last seven years, sucker.
What this means, practically, is that the problem is somehow my fault, but I don't know how to fix it. Until I can figure out exactly why this system works for everyone else in the world and not for me, I effectively have a store that can sell one item from one shelf, and that's not really going to work for me.
So, Occam's Razor says that there's something screwy with my server configuration, my html or php code, or something else at Monolith Press, right? There's something going on that's preventing the form at Monolith Press from passing all the appropriate information along to PayPal's order processing system, right?
Actually, no. When I export my account history to a CSV file, to check and make sure that all the appropriate fields are being filled in, the item number and description are right there, exactly as I entered them when I created the form.
So Occam's Razor really says that I am doing something wrong, but I really have no fucking idea what it is or how to solve it, and I have no idea who to turn to for help, now. This is the most frustrating thing in the world.