Address, city and state abbreviations should be avoided (ex. St. Ave., P.O. Box, CT, CA, etc.).
Proper etiquette indicates you should spell out: Street, Avenue, Post Office Box, Apartment, Unit, Connecticut, Vermont, etc.
When addressing a street address with the numeral 1 – spell it out (ex. One Main Street)
Addresses with numerals 2 to 20 can be spelled out, although it’s optional (example Twenty Main Street).
If possible, try to avoid 5 lines of addressing (the U.S. Post Office will appreciate it).
Title abbreviations can be used and are socially acceptable (ex. Mr., Mrs., Ms., Dr.).
Suffix abbreviations can be used (ex. Jr., Sr.) or they can be spelled out (ex. Junior, Senior); omitting the comma before Junior or Senior is optional and it is a personal preference by the person.
In the case where Roman numerals are used (ex. II, III); It is optional to also be spelled out (ex. John Smith, the Third) based on the actual typestyle chosen omitting the comma before II or III is optional and it is a personal preference by the person.