If I remember correctly, by law in majority of States, there's a 3 day limit on transactions where one can return an item at no extra cost.
Since the 80lbs version is the identical seat, you could always try it out for fit and then return it afterwards. Or (alert -- the not totally honest route
), they might let you try out an installation in your vehicle in the parking lot... And you tell them you're not satisfied regardless of the outcome. But at least you'll be able to know if it fits and if you like it.
I might go ahead and write Consumer Reports about the two seats being priced so differently, but still basically the same. Something that irks me as just not being 'right'.
They might like to investigate it. I got a really nice response from my gripe last week concerning an Apr 2006 article about larger car seats & obese kids. I told them that they were not helpful to the general public and that more parents want 5pt harness choices for taller kids because so many are outgrowing these seats at a young age. To say that only obese children needs these seats is to limit the category. They're advocates for safety and fair practice, so this Radian issue could be up their alley.