Sounds like you have set up two items, one for each UPC code.  The program does allow you to use one code for the product and set either by the case or by the unit.  In fact, you can buy by the skid, stock by the box and sell by the unit in the program.  This would be the ideal setup based on how the program is designed because it allows for reporting in each unit.

However, if you have it set up to us the case UPC and don't have the unit UPC when you sell, then you have to find it, which can slow down data entry.

To get your units and cases with two item codes, reporting under one, you will have to implement some custom reporting of some kind.  Eg.use one of the Additional Info fields to link the same products together and dump to Excel and merge the reporting using the features of Excel to combine the quantities.  There are other ways of doing it but they all require customisations outside of Sage 50.

As far as I know, there is no way to combine the reporting of two item codes together within the program.  The program is design such that two different item codes are two different items.

However, you could theoretically receive the product in by the most salable unit.  eg. you do more sales by the case than by the unit.  Then use item assembly or builds to split a case when you wish to sell by the item.  This would keep most of it under one item code until you sell, then you only move roughly the amount to be sold to the other unit.

Why is there no simple solution for this problem...I have just starting using Sage Quantum.

The programmers haven't programmed a simple solution for your specific scenario.  As I mentioned, Sage 50 is programmed so that one item code is one item, not one is possibly just another salable unit of a different item code.

The programmers designed it so that 1 item can have three types of units for the same product.

You can have it packed by the skid (10 boxes per skid), box (100 units per box) and unit (just one salable unit)

or

case (10 boxes), box (5 bags) and bag (50 marbles per bag - but you don't sell by the marble, you sell by the bag of 50 marbles, box = 250 marbles or case of marbles = 2500 marbles).

The programmers haven't programmed a simple solution for your specific scenario.  As I mentioned, Sage 50 is programmed so that one item code is one item, not one is possibly just another salable unit of a different item code.

The programmers designed it so that 1 item can have three types of units for the same product.

You can have it packed by the skid (10 boxes per skid), box (100 units per box) and unit (just one salable unit)

or

case (10 boxes), box (5 bags) and bag (50 marbles per bag - but you don't sell by the marble, you sell by the bag of 50 marbles, box = 250 marbles or case of marbles = 2500 marbles).