It's an option that I'm paying for on as yet unreceived DL. Does it include the rear seat back covers?
No, it's for your floor. Ask your dealership what that entails - different regions has different levels, since it's not an OEM defined package but a dealer/franchise defined add-on. Some may come with a trunk liner. My Prius C did, but not my Prime.
The all-weather liner package comes with four floor liners(2 front, 2 rear) and one cargo liner for the back. As far as I know, it does not include liners for the rear seat backs. There is a separate cargo liner set you can order that comes with the rear seat backs, but I don't think they come with the basic package. The cargo set I know of won't work in the stock LE unless you do the spare tire conversion - it's for the high floor/Prime cargo area. PT908-47235-20 (Ignore the title of the listing - it would work just fine in a spare tire car) 23-25 PRIUS/PRIUS PRIME (WITHOUT SPARE TIRE) ALL WEATHER CARGO LINER OEM TOYOTA | eBay
Don't let them not give you your cloth floor mats. The real scam is that you pay for the all-weather package but they sometimes try to TAKE AWAY your cloth mats which come with the car. You should get both if you buy the all-weather package and don't let them treat you like an idiot since now you know better. I had just assumed my cloth mats were in the trunk storage or something and didn't think much of it since I bought my car in the winter so I never went looking for them until dry skies showed up.