Interestingly enough if you can get enough 99.999s in your speed for the ship itself there is almost no time that passes at all, which then pushes the question of how fast can you speed up and slow down. If in this magical world it was quickly you could ship goods that have pretty short shelf lives without issues.
Even if you could somehow do this technically (without the ship blowing up when it hits a micro-asteroid), the fact only a short time passes inside the ship isn't much help when it takes decades or centuries for the ship to get to its destination. Even at light-speed with no accel/decel time, a ship would take 4 years to get here from the nearest star system (Alpha Centauri).
