I always wanted to get the opportunity to get into the UPS air distribution center in Louisville, KY or FedEx's in Memphis, TN. I was at a hotel next to the Louisville airport and could not count the UPS jets landing there were so many, the number of daily packages would be impressive.
All those conveyors tied together with hundreds of discharge points, multi-level storage, package tracking (in, out & moves), logging, etc. Those must be some pretty complex systems.