Short answer: The 2-bottle package carries a shipping charge while the 3 and 6-bottle packages ship free within the US — a specific, direct distinction worth understanding before choosing.
Why only the smallest tier carries this extra cost
Free US shipping on 3 & 6 packs is stated directly as a formula-wide benefit — worth noting the 2-bottle tier is the specific exception to this broader policy.
Why this affects the true per-bottle cost comparison
Factoring in the 2-bottle tier's shipping charge against the 3 and 6-bottle tiers' free shipping is worth doing before assuming the smaller package is genuinely cheaper overall.
💡 Worth knowing: promotional pricing is set at checkout and can change — confirm the current offer, including shipping terms, on the order page.