Let say we have an address of the following form (4 lines):
Apartment 5, Banana Residential Area,
No. 28 Apple Street,
[City], [Province], [Postcode],
[Country]
Now here are the questions:
Sometimes, the country is selected from a drop-down list, so I may forget to add the last comma.
Traditional British usage was to add a comma at the end of each line (except the last, which sometimes had a full stop) and indent each successive line. A comma was also often added between the house number and the street name.
However, indentation is unusual now, and the Royal Mail recommends omitting commas. It also recommends writing the town or city name in capital letters, and putting the post code on a line of its own.
This isn't a matter of English usage at all, though. Practice may vary between English-speaking countries. Further, if you were writing a letter to a French address, you would be well advised to format it in the conventional French way, and if you were sending a letter to Germany, you would address it in the conventional German way (regardless of whether the letter itself was written in German or in English).
There was a question in the comments: "For which country are you asking?" Each country is different. Here's an example from the USA. The formats might vary, so this isn't the absolute rule.
Jethro Tull
28 Apple Street, apt. 5
Los Angeles, California 90210
USA
In other words,
Name
Street Address
City, State Zipcode
Country
Where should insert a line break? For example, should I write "Banana Residential Area" in the second line?
It's unclear what "Banana Residential Area" represents. Usually an "Area" isn't part of an address. If it must be included, a whole separate line might be a good idea.
Should I have a comma at the end of each line? In particular, the comma after postcode.
No, don't add commas at the end.
Formatting varies by country and sometimes between the label and the letter itself (the US Post Office doesn't like periods or commas, except for hyphens in the zip code):
https://pe.usps.com/text/pub28/28c2_007.htm
On a enclosed letter I would write:
Jethro Tull
28 Apple Street
Apartment 5
Los Angeles, California 90212
But on the envelope I would use no commas, use CA instead. I always break the apartment/suite/floor onto its own line both in the letter body and on the envelope. This is not obviously necessary or attractive, or even preferred by OCR scanning, but if you've ever seen a rushed mail clerk trying to shove letters into two hundred mailboxes in a large office building or residence you realize it's probably a good idea.
