When I use a VPN into my office network and try to access Amazon or Shutterfly, however, everything's fine again.
I ran a tracert (w/o VPN) and found I was timing out after about 9 steps to get to Amazon.
Talked to Amazon. Amazon says talk to my provider.
Talked to my provider (Cox). They claim my dynamic IP is on the SORBS blacklist from 9+ years ago. I have a request into SORBS to be delisted, but I was under the impression they blocked email. This isn't an email issue; I can't login!