So my company we sell caps wholesale

Now when a customer buys 1 cap it's 7$

But if the customer buys 72 caps he gets a .50$ discount so each cap unit becomes 6.50$

Now if the customer buys 144 caps he gets an additional .50$ discount which is 1$ discount so each cap becomes 6$

You guys get the principal... Now I usually have 3 layers so I want to know how do I implement that so I dont have to manually do it each time I invoice...

I also want to know is there a way I can get it to work with more than one Item # so lets say he orders Item #3214 x 46 but he also orders Item #3222 for 50 it'll count the total of the caps and give him a discount or that's out of the question it has to be all the same item?