what are your order strategy for those goods?
by default, it waits until you are below X months of stock, and then places and order for Y months. I think the default is 4 months, and then it buys 9.
This is all configurable on the order strategy page from the main trade page.
If you want to have it buy in smaller amounts, or keep less of a stock overall, you can adjust the amount of months it attempts to purchase.
Alternatively, if you want to configure the specific amount of goods, there is a toggle for each item where you define a low water mark and an order amounts. Its functionally the same to the above, it just works on raw numbers instead of months.
For most of my weapons, I set the the total I want, and then set a much smaller order amount, so it can trickle in over time without spending all your cash in one go.