intrest charge

Mr john, have watched your free lectures on inventory control

The annual demand for an item of inventory is 45 units. The item costs $200 a unit to purchase, the holding cost for one unit for one year is 15% of the unit of cost and ordering costs are $300 and order.
The supplier offers am 3% discount for orders of 60 units or more, and a discount of 5% for orders of 90 units or more.

Calculate the cost- minimising order size.

my question is, how come the discount is only accounted for in holding costs and not in ordering costs too.