Heavier products do not qualify for the S.O.S. program and are subject to normal shipping rates. Products that require dimensional weight shipping or special handling, such as bottles, carboys and large pieces of equipment are also subject to standard shipping rates. This list also includes, but is not limited to, Wine Recipe Kits, any Italian Wine Equipment, Oak Barrels, and Beer or Wine Equipment Kits.
Saving with the S.O.S. program is automatic. If all items in your cart qualify, you will automatically receive the Save On Shipping rate. There is no need to enter a promo code or anything else to receive the lower shipping rate. Cheers!
So you only get the flat rate shipping on light items; tanks, carboys, barrels - all the stuff you'd want cheap shipping on, use standard shipping.
Aside from carboys and barrels, it doesn't really say what qualifies as "heavy"