You might be able to build a small "chute" on the stairway, we do it all the time in construction. Put gravity to work, just be careful gravity can do a lot of damage too! We normally build them out of plywood in a "U" shape, with bottom being more squared. They can be square, round and triangular shaped, built as modular units (sections), transported to a site, assembled in hours on site. There are even prefabbed metal chutes, can be rented, companies that deal with scaffolding will usually have them.
Just be careful, moving things down stairs manually can be dangerous, the more repetitive and greater the weight, will increase the risk of injury or damage to property. Think it through, is there enough potential for financial gain to offset the risk and liability. Hate to see you cause problems for your full time employer over a small gain on the side. I would ask myself, about why this manager is willing to risk your job for mostly his gain!
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