May I be so bold as to partly not agree with your solution.
Maybe, it is just a subtlety that makes my wrong choice wrong, but your description of the solution is to me the count of orders. If you tell me to get the count of customers, I would always retrieve the count of distinct customers. I do not see any point in telling my business that there were 5 customers having paid with debit card, when two of the orders were paid by the same customer. I am not entirely sure that a single group by would suffice to count distinct customers (in SQL I know I can, but in KNIME I miss the “distinct” checkbox of the count aggregation).
The choice of yours I did not tick, to me, can be done easily with a single group by. As it is the aggregation over two columns, I would prepend a math node to equate the sum of the two columns into a column of its own. Then, there is no problem to use a single group by. I, actually, was not able to get my head around to a solution with a group loop.