If you live in a rural part of the United States, you might use a post-office box as your billing address, since it’s a more-secure destination for important letters such as bills, and it can be usually differentiated from a shipping address that you point at your home or office, depending on what’s most convenient.
But as TidBITS reader John Hinckley, who lives in a small town in Vermont, pointed out to me, those who do rely on a PO box may have trouble ordering an iPhone from Apple, because Apple won’t ship to PO boxes. The fact that you’re not shipping to such an address is irrelevant because, at least in the case of the iPhone, Apple reportedly picks up your AT&T billing address as the default shipping address. Presumably, the same applies to Verizon Wireless and Sprint customers in the United States; I imagine the situation may vary in other countries.
When John called the Apple Store to resolve the problem, the sales rep said she was unable to help, but she had spoken to a number of customers with the same problem. Luckily, he hit upon a workaround, which was to change his billing address on the AT&T Web site temporarily and then place his order for the iPhone 5. Once it arrives, he plans to change the AT&T billing address back to his PO box.
If you run into a similar problem, John’s workaround is worth a try. It might also be worth trying to order from your carrier directly to see if they will let you differentiate between billing and shipping addresses. I ended up ordering from AT&T, not because of any address snafus, but because AT&T was still promising that my iPhone 5 would arrive on 21 September 2012 when I placed my order early on the first day of pre-orders, whereas Apple was already saying I’d have to wait until some time in October.