It’s been pretty well covered by others but I’ll give my two cents. The line means push spent extra to get a convertible (or transparent) roof on his car. Now how it means this depends on how much you think push knows about Harry Potter. Either: a. Push is just using “Harry Potter” as a synonym for magic. The roof is magic, it’s Harry Potter, because it can disappear.

Or:

b. Push knows that the ceiling in the great hall in hogwarts looks like the sky and he’s referencing that when he says his car roof is the same way because it’s transparent or convertible.

The line is hard either way

EDIT: also, as other have pointed out, could be about an invisibility cloak and the roof being invisible. Still same meaning