Sometimes we need that bit of extra room and that sometimes was the inspiration for the Expansion Pak.
Simply slip it under either the tail pack and secure it with the A or AA Bagz straps through the loops on the Expansion Pack.
Use it for spare shoes, toiletries bags, towels, jeans, camping gear. I use mine for all those things I don't need 'til the end of the day. The zippers on the end allow me to tuck away a neck warmer or spare gloves away quickly.
Expansion Pakz fit neatly inside (on the bottom) of the A Bagz if not in use, but might be needed for the shopper of the team.
My favourite combo is the AA Bag and Expansion Pak. It fits neatly between my panniers on the seat.
Use the Expansion Pack as bag in its own right. Top a hard case, use it for tents, camping gear or wet weather gear on day rides.