Try with min(ship to days) and check is it correct or not.
Thanks for your effort, lets say hypothetically (to relate it with my dataset) if the order number is repeating for e.g.
Order # abd123 is in Jan 2018 and ship to days = 5 days and it repeats in Apr 2018 and ship days = 10 days then since we have used min() it will return 5 for both cases month .. Is this making any sense to you?
you know your data - the order could have multiple shipments (i.e. not all item shipped at once) so you wuld need to decide if you wanted the min(), the max() or the avg()
would depend on the business decisions being made