You can install our site as a web app on your iOS device by utilizing the Add to Home Screen feature in Safari. Please see this thread for more details on this.
Note: This feature may not be available in some browsers.
So I am just playing with ordering an S at 10:10 am PDT on 5/17 and see the purchase price as $78490, but upon reopening the webpage it is back at $79900.
It's likely that the $78,490 price you saw had the $1,500 California Clean Fuel Reward applied. Without that, the retail starting price is $79,990 ($81,190 with destination and doc fee).