Many choices...the best would be to reset the flange to on top of the finished floor. Then, yes, there are flange extenders. There are also some waxless seals that have a funnel like piece that would take care of that (Fluidmaster and Fernco are two brands). Or, you could use two wax rings, one with a funnel, one without. The one with the funnel on top so the funnel helps keep the bottom one centered and so it can't smush into the drain to create a blockage. Normally, a single one with a funnel can be more problems than it solves, but when things are low, it can help.