I just had need to use these showers last week and thought I would provide an update to this thread.

We identified two sets of pay showers. The first set is at international baggage claim - before you pass through customs. There are two shower rooms down near Carousel 6 but you need to get tokens at the Swissport desk near Carousel 2. It's 15 SF for the token and Swissport only takes cash. The showers are adequate but no amenities or anything of the sort.

The other set of showers is outside of customs. You can get a token at the Swissport information desk outside of customs (Gold Desk). The showers are to then to the right down near the post office.

There were three in our travel group and we had all planned to use the showers outside customs but because there were only two, one of us was admitted back through customs to use the showers near Carousel 6.