We find that sending flour through the USPS is the most cost effective way to send.
Priority, Flat Rate Mail is best.
Our 3-pack (6.6#) and 5-pack (11#) fits in a medium flat rate box for $10.70
Our 8-pack (17.6#) $14.95.
On the west coast, Gail always checks UPS... sometimes it makes more sense to go UPS.