I only send them for my own collection, so I typically send 2, 3 or 4 (depending how much I like the pictorial postmark) so if it gets messed up in transit, I have a second one to fall back on.
As luck would have it, I sent these without a cover envelope and so they were returned through the general mailstream. I always put a piece of removable tape at the bottom 1/4" of the envelope along with a return address so the bar code gets imprinted on that rather than the cover, and I can remove it when they arrive back in my mailbox. In this instance, the second cover went through the bar coder upside down, so I have the bar code on the top of the second example.
Oh well, I try, but as always, we're at the mercy of the post office in getting these covers back to us safely.
I know some will comment that I should have included a self addressed stamped envelope with my request for the safe return of the covers, but it is not required per the USPS Pictorial Cancel Policy, so I try to save on postage by not including one if the postmark isn't that vital to my collection. I have found that 80% of the time, I get them all back in good condition without including a SASE, with some post offices even being so kind as to return them in a USPS envelope. Of course, there are always the other 20% that get put through the canceller and I get a messed up cancel in the end, but they are the exceptions rather than the norm.
In fact, here's another one I got (through the mail without a return envelope) from Des Moines, Iowa, that pulled Mr. ZIP out of retirement for a ZIP Meets Date cancel, supplemented by a Motorcycle Awareness postmark as well. An interesting dual purpose cancel for ZIP 50312: