It's simple:
Step one: Take an Ammo Rack
Step two: Take a horse
Step three: Stick ammo rack into horse, horse now dispenses ammo, but is super slow due to ammo rack in its derriere.
Step four: ride horse along with your army, they get ammo.
Step five: Make upkeep for this very expensive.
Step six: Go masturbate?
Step seven: Reduce max amount of ammo archers can carry, make them dependent on supply lines.
Step eight: Create a horse called "Wooki's Whores" thats a courser with +1 speed that only Wooki gets for this awesome suggestion.