I want to second Penlowe's question and ask some more.
1. Do we send it in an envelope or just straight into the post box?
2. The swap info says you must use "delivery confirmation" or package tracking of some sort when you mail your package. Do we have to do this seeing as it is a postcard?
3. Do we write on the back or leave it blank?
Sorry for all the questions!
1. That's up to you. Some people like that and some people don't.
2. No DCN required at all.
3. You write on the back. If you want to trade extra postcards you can leave those blank.