It's not that complicated.

A store sells its products every day.
Some stores offer everyday low prices.
The everyday in the second sentence is an adjective, you know, like "low". This is a very simple grammar rule, yet dozens of leading companies can't seem to reason it out. Today I was at Ross and their wall banners proclaimed, "Ross offers low prices everyday". Wrong. This CVS commercial, which is probably running nationwide proclaims "flu shots everyday". Wrong. It should be "flu shots every day". Companies like CVS pay ad agencies big bucks to create commercials like these. They're only 30-seconds long. You'd think they could get them right.