Hacker News Reader

The discounts on shipping you can get for buying multiple items from a single vendor make the objective function non-linear.

Integer programming. Which makes it a lot more complex.

Gurobi is sloooooooooooow, and scales pretty badly.

What's better than Gurobi? I used it at a previous job and we weren't aware of anything better (but it also seemed slow and not great at scaling).