They might have had deferred revenue in terms of warranty. They probably have to allocate a portion of each sale as warranty and recognize that revenue over the length of the warranty (similar to iPod). Problem with that is it means they would have had defer a portion of revenue this months sales as well... so this theory only holds true if they had sold a large number before this month.

Maybe OEM accessories and upgrades (like the Sport Package). Also they might sell a license to an after market shop so they can say they are the official Tesla parts brands.