I still needed to mail the card that I planed to drop off at the LAX airport in Los Angeles. There are plenty of US Mail drop boxes on the street in San Francisco. Since the forecast was overcast with a slight chance of rain, I decided to look for a post office so that the card wouldn't get wet. Google Maps showed a post office at Union Square, on the way from my hotel to the conference site. However, it took me a little effort to find it as it was well hidden in the Macy's basement.
If you enter Macy's main store from Stockton Street, turn left through cosmetics, take escalator down to the basement, turn right through the foodcourt, and find the cookware section, you will see a small "Post Office" sign on the far side with the entrance invisible on the right. If you enter from Farrell Street, take the stairs to the basement and walk all the way to the cookware section on the other side.
It's a small yet full service post office. I came during the lunch hours with little waiting time. There was to be a postal rate increase effective the coming weekend. The staff was very knowledgeable with the details and I was able to save a few nickels by purchasing some USPS Global Forever stamps.
It had been one of the only few United States Post Offices that opens on Sundays. However, effective May 4, 2015 this post office only opens Monday to Saturday.
Outgoing postcard US-3373244 to South Africa shows the Tony Bennett's heart sculpture at the corner of Union Square. The picture below shows the heart-shaped sculpture in its current form, near the Macy's store across the street.