Well, as it turns out, it wasn't quite a postcard. As usual, I just plunged right in and didn't follow any directions except for the ones that rattle around in my own head and I made a card that was too thick.
So it actually was not a postcard, and not even a letter, but rather a parcel.
So my sister will be getting a housewarming parcel.
It's official, and now everyone at the post office knows it, too. I'm an idiot. But at least I'm an enthusiastic idiot.