Hello!
The answer is: Mitchell can order a maximum of 5 CDs.
Why?
Let's make the entire shipping cost apart to calculate the actual money amount Mitchell could spend just buying CDs,

So, he will be able to spend 90,1 $
In order to calculate how many CDs Mitchell could buy with 90,1$, she/he should divide the total available amount by the CDs price which is 15,99$
Doing the calculation we have:

So, since a CD is an indivisible object, Mitchell can order a maximum of 5 CDs.
Have a nice day!