Lots of good answers here. I've seen almost any type of paper used with handwritten addresses or stick on labels. Most make it through with little or no damage.
Here is a link to Canada Posts standards http://www.canadapost.ca/tools/pg/st...SMach_SL-e.pdf long and complicated, pages 16-17 & 22 apply to postcards - your local postal service will likely have similar standards available. Basically anything with an address on the right side & a stamp seems to make it through. So why not join us and give it a try.