One thing to remember in considering bulk, even with a large flock, buy only what you will feed in about one month. Nutrients degrade with time.
I see what your saying, but if you have an account then you don't have to buy it all at once. You can go in every other week and grab 3 or 4 bags on your account. You can still get a discounted rate as a high volume buyer that way, and your feed never gets old.
Only problem with that is your parents will get billed monthly so they will see how much 90 chickens is costing them lol