It all depends on if the machine will read the address or if it gets spit out. Technically, the PO will charge .20 non machineable surcharge, but 9 out of 10 will make it without the surcharge. If you go to the window, they will charge you the extra. If you put it in a collection box, your chances of it not being tagged for postage due are greater. If it does get spit out from the machine, your chances for postage due increase. The best way to address these is with a sticky label (Avery 5160 or others) or putting the address on the inside.
The machines are getting better at reading through the plastic if you do put the address on the inside.
If the envelope is square, do add the extra postage. When in doubt, do it right the first time and get your card there in time.
Because the envelopes are plastic, they are a little more slippery than a paper envie. The glare of the plastic, makes it more difficult for the reader in the machines to read.
Have fun and be creative!