Stable. Able.
Blackburn developed their Outpost Elite Universal Seat Pack (USP) to work with dropper or standard seatposts, be able to carry a bevy of cargo, and be stable enough to not swing around while it does. The USP is a two-part system: Part 1 is a T6061 Alloy Wing that affixes to your saddle and works with a standard or dropper post. Part 2 is a removable and waterproof dry bag. The bag has a scuba-style air bleed valve to assist with compression while off the bike. The wing stays on the bike, the bag comes off and allows you to pack, compress and go ride in the snap of a few buckles.

Details:

Installation
Wing mount keeps your load from swaying during pedaling

Water Resistance
Welded waterproof dry bag

Capacity
Holds up to 10.5lb/4.76kg

Compatibility
Works with standard or dropper posts

Weight
572 grams