The unit is usually formatted as either `{building number}-{unit number} {street name}` or `{building number} {street name}, Unit {unit number}`. But both resolve to the same thing.

What I like is that address box 2 (sometimes unlabelled, lives under the address box) exists specifically for unit numbers. So, I'll put in my street address in two lines:

  Street Address  :792 Charleston Avenue
  Street Address 2:555

  Street Address  :792 Charleston Avenue 555
  Street Address 2: