Since the city list is not long, I briefly cleaned the list by hand, and stored it in ptoassigneend_us_citylist2.
* Actually, we can buy a full list of U.S. cities online: https://www.uscitieslist.org/. Just update the ptoassigneend_us_citylist and follow the following steps.
Then, similar to Section 4.2, identify clean data that meets all the requirements: postcode_extracted with 5-4 or 5 digits, state not null or not spaces, and city_extracted in ptoasigneend_us_citylist2.