The BAC discount is already built into the pricing on those long ducts. Those ducts were designed specifically for the mid-1970s and later 19/23/24, including the adapter cuff on the 132" ducts for the 1977 and later 19/23/24. With the original price already quoted expressly for these BAC ducts, Jon can't afford to then apply yet another discount. The regular BAC discount is for his standard pricing on general stock offerings to all retail customers.
To put this in context, Jon is providing very high-quality, custom-terminated SCEET duct to BAC members for slightly over $8.00 per foot. If you would prefer to buy the duct from Aircraft Spruce, and come up with your own end adapters, you can get it for $7.40 per foot (not counting your cost for end-adapter material). So Jon is essentially charging BAC members a premium of about $10 on the 132" ducts.