I believe you still use the two letter abbrev. I do think though that for St. Ave, Rd., etc that these should be written out in full. Street, Avenue, Road.
I totally stressed about addressing...here are some things i figured out that might be helpful...mostly courtesy of 'crane's blue book' and the 'bride's book of etiquette':
Spell out New York, New Jersey etc. also Avenue, Road and Apartment. Do not spell out apartment numbers but spell out street numbers less than ten. i.e. 2 blueberry lane would be Two Blueberry lane. Also, you can follow this rule for manhattan addresses or spell out all 'street' names under 100 i.e. 33 west 33rd street would be 33 West Thirty Third Street or 33 West Thirty-third Street (either one is correct).
