The 2-bottle package carries a shipping charge the 3- and 6-bottle packages don't — worth doing the actual math before assuming it's the cheapest way to test the formula.
The 2-bottle package is $79/bottle, totaling $158, plus shipping — while the 3-bottle and 6-bottle packages both include free shipping, per this site's own pricing breakdown.
Since this site describes most users noticing a difference within 2-4 weeks, worth weighing the smaller package's added shipping cost against the free-shipping value of the larger packages before choosing.