Okay, so I screwed up. I needed stamps, the local post office I knew of is closed during my work hours, so I ordered some on Amazon. When they showed up, I realized that they were postcard stamps.
I could send them back, but that would be a hassle. Can I send a regular letter by putting two of them on the envelope? Is there any other way to use them? (I don't see sending postcards any time soon.)
P.S. I am not at all sure this is the right kind of question for this site. If you know of a SE site where it would fit better, feel free to send this there.