modelling warehouse picking
My first IT job was in this area - basically I was involved in a "fiddle" on the part of my employer to land a big contract which involved convincing the client that we had a functioning and "intelligent" order picking system. It involved alcholic beverages and there are many complex rules about these (tax reasons).

Anyway, I just wish that Open Source had been as prevalent then as it is now - because there appear to be a plethora of high-quality projects out there in this area. A quick Google landed me here and here - search for the word "pick" on these pages.

If I were you, what I would do is examine these (and others...) packages and see which ones correspond most closely to your needs - download and install them and then check out the underlying schemas and see which bits you can borrow for your project. Or you may just decide to implement one of the systems that's already out there? 

What I do know is that the logistics behind these problems are very complex (operations research, tricky mathematics &amp;c.), so you might be able to take some of the pain out of your project by "standing on the shoulders of giants"!

[EDIT] 

You might like to check out this and its parent site- it specifically mentions bin-packing and employee rostering, so it might be of interest. I remember this from studying operations research. If you work in Java, it could be of interest - although the principles obviously apply to all languages.