> This is no longer true, I believe (starting Dec 1 2023). You can buy the vignette online starting immediately

If that's true then it's possible that Asfinag (the toll agency) haven't updated their website. On trying a test purchase just now to buy a two-month or annual pass it still states:

"I'm a consumer

Digital 2-month vignettes and digital annual vignettes purchased today are valid from 25.12.2023 at the earliest due to the right of withdrawal when purchasing online. All other toll products can be used immediately. (More info in the FAQ)

I represent a company

The right of withdrawal does not apply to commercial customers; purchased digital toll products are therefore valid immediately. (More info in the FAQ)"