I highly recommend FilterBuy.com
Filterbuy furnace filter fits nicely in the box. The actual filter dimensions are slightly smaller than what is advertised, however, that information is disclosed upfront. The corrugated membrane that is the filter is think and has a decent amount of pleats. You'll notice the larger the filter, the larger the pleats and also less pleats - than, say a 2x16x20. Overall the filter installed easily and the build quality appears to be good.
Since the act of changing furnace filters is predictable and frequent, I decided to make life easier on myself and have filters automatically shipped every few months. Setting this up online is simple and quick. Filters appeared on my doorstep within the interval of time that I set. Filterbuy's custom service is outstanding! I had an issue with the auto-shipment feature and sent custom service an email. Within a day, a representative had reached out and had already begun resolving my issue. I had a resolution within 48 hours. I didn't have to worry about sending the filters back (wrong size), instead, the rep encouraged me to donate them.
All in all, I am impressed with this company. They provide a quality product at a very reasonable price. The online shopping experience is easy and intuitive. Custom service is responsive and focused on resolving an issues quickly and easily, not requiring customers to jump through hoops! I highly recommend FilterBuy.com