I just went to Audi USA's site and configured a car, but I didn't see a place where I could choose rims ... or the S-Line package, for that matter.

I went with the sport package -- mainly to get the sport seats, and it says it comes with 18" wheels, but didn't show them.
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You can see all the photos and wheels (limited by trim selected).

In the Build Your Audi configurator on the right, you have to select a engine model with the Prestige trim before S line option will be revealed. S line only has one 19" wheel available. The sport packages come in 2 wheel flavors - 18" and 19" - the Premium Plus pakg is required for the 18" sport option and the Prestige offers choice of either 18 or 19".

Wheels are basically determined by trim - Premium, Premium Plus or Prestige.