One day pass works on multiple flights. If you’re pre-buying it on delta.com you should be able to uncheck it from the second flight so you’re only charged once (I did this last week and my single North American day pass worked across 2 flights). Not sure why Delta does it this way, I’m sure it causes people to overspend

I’ve also found that despite Delta’s terms saying that the WiFi pass is only good for that particular flight, the pass is stored with GoGo and can be activated for a flight any time within a year of purchase. (I’ve had flights where I pre-bought WiFi then didn’t end up using it, and was able to log into gogo and use the pass on a flight on a later date)