So it does sound like how I described. What a scam! Buy the first half of the game and then have to buy the second half after waiting for you to have it mailed to you.
It really wasn't too bad. The games were seriously a dollar or two, just the cost of the media it was delivered on. If the game was awful, you had the floppy, for sometimes less than new floppies cost at that time. Honestly isn't really much different from downloadable demos now, except that there was no way to connect and download the demos back then. They had to give you something physical to provide exposure to the project. Most shareware was made by someone who just was transmitting it themselves.
Think of it as if it's a new band getting started (pre-Internet). They sell their demo CD at their shows for $5, and it only has 5 songs on it. You are only getting half a CD, but you are getting something. A lot of times, these little games stood on their own, but the full version of the game provided more characters or levels. It wasn't really like demos we see online today, where it's timed or whatever.