I was only partially correct. Looked again at asfinag and the grace period does not apply to 1 and 10-day vignettes. From their website:

"18-days-period Consumer protection is very important to us – especially as far as our digital products are concerned. Customers can withdraw from the online purchase of a digital vignette within 14 days. Taking into account a further three-day period for mail, your digital 2-months and annual vignette is only valid from the 18th day after purchase.

This deadline does not apply if you purchase a 1-day or 10-day vignette!"