The premise is that there's a guild of couriers that use a dangerous parallel spirit world for shortcuts through mortal space to deliver packages at great speed.
Despite the advantage of speed, why would customers choose them over classical shipment? Given the dangers the couriers put themselves in, plus dedicating a single individual to at most a few packages at once, the service would be quite pricey.
While their service would be faster, what could be a reason they are hired when a supernatural being needing a quick delivery could simply opt for, say, shipping via overnight UPS?
Some added detail:
Dangerous as in "unless you're trained to do this job, you will probably die."
Fast as in within hours, maybe even under an hour. A courier might make multiple hops between the real and parallel world to plot the most efficient path, and this could lead to very quick routes.