Delivery Service fills the city with more hanging barrels you can shoot down on enemies. Throwing knives can also trigger these traps.
Delivery Service is entry 4 of 9 in the Insider branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £1,000, 50 Leather, 25 Silk. Before Delivery Service unlocks you need Little Rooks.