First, I'm not sure what happened to the WMS once Macola purchased it from IPS and integrated into their later version of Macola. I use Macola v7.5.103d with IPS-SAS version 7.5. I think much of Macola's WMS is still somewhat similar to SAS, but I just want you to know what I'm working with. When I refer to SAS below, I'm referring to the WMS.
Given that, I believe you are experiencing a buggy little feature that apparently Macola has never dealt with; that is, backorders that use the same order number as the original order. Table OESHPTCL still contains the cartons that were shipped with the first order and this is why you get the overship error. With my version, you must delete the entire order by clicking on the 'telescope' speedbutton in SAS and calling up the order number. You then right click on the order number that displays and choose to delete. You can then re-release the order from OE and only the backorder records will be added (if you've posted the original order). Be sure to telescope the order again, right click, and re-calculate. However - and I emphasize that - sometimes, especially when OESHPTCL get rather huge in size, the delete feature I mentioned doesn't always work for some unknown reason. In that case, I ODBC the table and run a SQL delete query. You'll never complete this order unless the records are removed from OESHPTCL for that order. Another way to delete is that you can also telescope the order and when it displays you can expand it by clicking on the plus sign. You can then right click on a carton and delete it. This isn't a practical solution with huge orders - or if you require new carton numbers.
I had to write a utility program that works around the many pitfalls for the version of our WMS, and there were many. Not to chide the original programmers, as they were very good and also very nice people. Our shipping personnel can now easily resolve the problem you are having without having to call me. There was also never a purge feature that would remove records that weren't associated with open orders. The OESHPTCL table grows astronomically in a relatively short period of time. I had to write a program that would do this and have to run it about twice a year. It takes care of all the SAS tables and the shipping department smiles more often.
Getting answers before I'm asked.
Providing answers if I can.