Isn't this a classic EOQ problem right out of Finance 101?

The classic EOQ (Economic Order Quantity) model assumes constant demand, but you can change your demand figures each time you run the model. You provide the model with your inventory holding costs (usually just your cost of capital unless you rent varying amounts of warehouse space as your stock levels vary, your fixed costs (typically your shipping and handling costs, your cost of preparing the order including arranging for and sending wire transfers, etc.) any quantity discounts that you can take at different order quantities.

I vaguely remember from MBA school, variations on the classical OEQ model that included both variable demand and an assumed stock-out cost. All this should be readily available with a Google search.

Combine this with what Bruce has already suggested and you should have some interesting approaches to your order quantity issue.