Not sure if this is what you are trying to do, but on the Customer tab on the Products record, you can for each Customer specify different packings and packing capacities. These are then used in the Order, Delivery and Packing functions. In Packing the total quantity will be split depending on the capacity specified.

There are 2 possible solutions: using a single product code with different pack units or 2 products codes.
To me the key questions to ask to pick one or another:

1. Do they have the same value in you GL? if not, then better to have 2 different product codes
2. How to track the packing activities? A unit change is simpler but not possible to schedule the load or track the cost. On the opposite, with 2 differents product codes a work order can be useful if youconsider this is part of your production process. An assembly can be a good balance between accurate costing and time entry.
3. What is the stock available that the MRP should consider? What should trigger the packing process?
4. Are you relying 100% on automatic allocation? If, not you have the risk of sending the wrong pack unit to the customer. On the opposite, with 2 differents products no error can be done.
5. Do you have customers that can accept A both in 72/case and 4/case ?
6. Impact on the volume of the product base: if all products can be sold in 20 different units, this will increase the number of product record to handle;
7. Do you have scrap in the packing process?

I hope this would help choosing the right solution.

The corrugate boxes are not part of the BOM or routings as they are expensed as warehouse expenses.  They only produce the each and the customer orders in eaches.  So Walmart will order 4 and they know that is a Walmart case.  So:

1.  They do have the same value in the GL. 

2.  Since they are already in cases, there isn't much packing needed except for putting cases into a pallet, but X3 doesn't handle that great either. 

3.  I am not sure they would use the packing process. 

4.  I would like to, if I could have a customer pick from a designated location it would probably solve this.

5.  No

6. I am not sure about this. 

7.  No

I don't really understand this function as they are already in boxes, but I will test it out. 

So actually we ended up using the customer specific translations on the item master and that gave us the result we were looking for.