Weird iTunes Problem

Ok, I'll note that I've tried this on two Windows PCs, a MacBook Pro, a Windows laptop, and a G4 Mac Mini, using versions 6.0, 7.0, and 7.0.1 of iTunes.

If I try to purchase a track in iTunes, it takes me to my credit card information page and asks me to verify my 3 digit CID code from my CC. No problem there. However, when I click submit, it tells me the iTunes store cannot process my request, and to try again later.

Okay, so I tried it on several other computers over the next four days; same thing. Try a different credit card. Same thing. Tried Paypal. Same thing (after Paypal approves it).

So, I go all out tonight and buy a $15 iTunes card at Walmart. It accepts it, I see a $15 credit. Try to download a song, and back to the CC billing confirmation we go!

I've emailed iTunes support, but they still haven't responded several days later, and I can't get through on the 1-800 number.

Any thoughts anybody?

Update: Apple just sent me a canned response telling me to try a different credit card.

Well, Apple has told me several things now, including the fact that my firewall (I don't have a firewall) is the problem, and also telling me now that the USPS is to blame (my zip code, which has never changed, is causing the problem?).

I guess no one has any other suggestions.