What is the deal with replacement filters for the very popular Coway AirMega air purifier? Prices are all over the place, from $32 at Walmart to $70 at BBB, with many, many reports of stinky chemical fumes. The ones sold directly by Coway are $57 which seems outrageous for a machine that I paid $150 for but maybe that's the only safe way to replace the filter? Help please!