When I "add a field" under the "Tips and advanced options" link, it shows up in front of the street address? Why? (e.g., Apartment 123, 100 Main Street, City, State, ZIP)

When you "Add a Field" such as a P.O. Box or an apartment/lot/trailer, that field will be displayed in front of the street address because it's a custom field. The formatting may look foreign to some people, but the US Postal Service is still able to ship items using this address format.