Formula to Sum Range that has the same value in another cell

A pivot table might be the easiest. Select the range -> insert pivot table -> instruct pivot table to use sales order (or customer) for the row labels and sum of units for the pivot values.

If you must use a formula, then something based on a SUMIFS() function would seem appropriate.
1) Build a list of unique entries from sales order or customer. The UNIQUE() function can do this, if supported by your version.
2) Adjacent to the list of unique entries, a SUMIFS() function like =SUMIFS($D$2:$D$21,$A$2:$A$21,reference to identifier)

Which strategy do you like?