I used to buy the fragrance products you clipped to the filter in your homes air return filter to help freshen air being circulated. They worked well – but only for a very short time and I quickly realized their cost didn’t justify their benefit. I found a cheaper way to freshen the circulated air.

I’ve been using a small amount of Carpet Fresh Quick Dry Foam. It’s advertised as not leaving a residue and being safe for use around children and pets (other than birds) ***when used as directed. It comes in several scents – I prefer Morning Rain. A can costs about the same as the other product but will last a long time. It only takes a very small amount to spread a clean scent throughout your house.
My return vents which hold the filters are located inside my home, not in the unit, and I use disposable filters. I wouldn’t advise using this on filters located directly at the unit . and please see the disclaimer below.

***Disclaimer – The product container states to “use as directed” and using as a vent fresheners is not listed. Use product in this manner at your own risk. I am not liable for any issues that may occur from using this product in a manner other than as directed on the label.