I realize that air filters are the perfect candidate for a subscription service... but it's just insane the that every product in existence has a business prepared to ship some of it to you monthly.
Do filters have a shelf life? If not, or if it's sufficiently long, I'd rather they just send me six filters once a year, instead of one every two months, or something like that. I get that some people might not have enough storage space to pre-order like that, but it's otherwise completely unnecessary to send one at a time.
(The only plus I can think of to the one-at-a-time model is that getting a new one in the mail is a nice reminder to replace the filter. But I also have this thing called a "calendar" that I can use.)