What are the sizes of the furnace and condenser?

I assume the 16X20 return was abandoned because it used a wall cavity instead of a duct. It is hard to judge the effectiveness of the two 20X20 returns without seeing the set up.

I would be leary of a tech who told me I should be using a cheap 99 cent filter on my new Carrier Infinity furnace and condenser. A good 1 inch pleated filter with a Merv 8 rating should work. Do not use the 3M Filtrete filters. They are too restrictive.

You may have a static pressure problem. The change filter message is triggered by the static pressure reading. It is too high, then the furnace intrepret this as a dirty filter.

You can go into the advance menu and get the static pressure reading. You want the reading when the furnace or condenser is operating in the high stage. The ideal reading is 0.5 or less. A reading of 0.5 to 0.7 is accepatble. Anything higher is not good.