I don't usually include cardboard in mine, but I use A4 envelopes. If you wrote D Not Bend on it then it shoudt get bent, but I know of caes where even when this ha been written on it has been bent.

What size is normal? If you mean the 8 X 4's or whatever your letter shouldn't get bent, but it does depend on the postal service in your area and things like that, but I'd say it's a 10% chance that it will get bent.
Hope this helps,
-BBCZ

I sent out a normal letter size envelope with no cardboard- will the photo come back bent?