can you just bring customer id to rows and order dates to columns?
Please use this post. It will help you.
Attaching the same workbook here.
Just download the workbook and try it. Check the third sheet.
Formula for you would be
PREVIOUS_VALUE("") + IIF(INDEX()==1, "", ", ") + ATTR([Order Date])
in Detail of Marks add Order Date,
and a filter like below
Filter to show only True
Then edit the last column i.e marks column to be hidden.
Hope this helps.