I found it...

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I stamped it, and I sent it. Somehow, the envelope I was looking for got stuck up on a shelf amongst my envelopes/bags of stamps and address labels. I'm not sure what prompted me to look there. In my last post I talked about quitting (as far as looking for the envelope I had wanted to send) but even then it still bugged me. I guess in my mind I was doing one of the "retrace your steps" things, which I rarely do because it even more rarely works, but anyway... The envelope went into the "outgoing mail" slot, though it won't go out until tomorrow because the mail has already come for today.