I've been getting Sunday deliveries for about a half year now. I too thought it was a mistake on my account page when I saw it the first time. I was actually surprised it was with USPS, because I figured their union would have nixed working on Sundays.
I have to say, I'm quite impressed with the USPS/Amazon partnership. Amazon has been using USPS for a lot of my home deliveries lately, and stuff always seems to show up as fast or faster than with the commercial carriers. It's a thousand times better than when they were using OnTrac to save money shipping stuff. I had about a 50% failure rate with shipments from them being late or delivered to a neighbor, especially around the holidays.
It looks like the USPS Sunday option is only available around the larger USPS hubs. I've never seen it happen for deliveries to my folks place, in fact the default is usually UPS to their address.