What exactly are the Flavor Cards? Is this the only flavor they serve during the week indicated on the website, or can you choose from other flavors?

They've always got Chocolate and Vanilla, then the flavor of week from the Flavor Card. Sometimes they'll have some of another weeks flavor left over. I don't know if the small batches are the reason it is so delicious or not.

Flavor cards are what they are serving for the week.

Usually only the three choices like Benjamin explained, but it's oh so worth it!