If I remember correctly, they carry more fuel than they need in the first stage as a safety reserve, and the plan is to use that reserve fuel to land the stage.

If I remember correctly, they carry more fuel than they need in the first stage as a safety reserve, and the plan is to use that reserve fuel to land the stage. So they actually don't pay an extra weight cost, they simply make use of margin they were including anyway. If they end up having to use their safety margin, then the rocket can't land, but in the majority of flights where things run smoothly, they get to land it.