Amazon claims that air filters of sizes 10x20 and 20x20 and 30x20 are frequently bought together.

I am prepared to seriously doubt that. How many different-sized furnaces do people have?

Which means that the "frequently bought together" phrase is bunk. There may be something that's "frequently together", like they showed up in search together, but "bought" it ain't. (And if it is search, their product categorization is seriously flawed, too)