Does this bubble envelope need to be stamped on the back by post office?

I bought bubble envelopes at a post office and noticed that they stamped the back of it with a stamp that said the date and what town post office it was from. I went back to get more but they did not have anymore. I read that places like CVS had some. When I bought them from there they obviously were not stamped. I have been shipping things without taking them to the actual post office for a little while now. Do the envelops that you get from the drug stores need to be stamped by some kind of post office and is the shipping label all I need?