There was a time on this forum where one of the master plumbers highly-recommended using spacers, provided you sealed between them with caulk (even if the instructions said you didn't need it), and from my reading there is still a cadre on here that thinks they are the way to go. I just reread a post on the subject from like 2008 from a plumber who had installed literally thousands of toilets, in which he said that that is the way to do it.
The more current teaching on here is Terry's method, which is just to use the two-wax-ring method. I have seen him say quite a bit that in his experience, the spacers often just lead to a leak, and that he is much more comfortable just dropping in two wax rings. If you're really down 3/4", that's no small amount, but a wax ring on the bottom, with another wax ring stacked on top, should probably do the trick.
I notice that there is also a divergent opinion on here about whether or not to use a ring with the built-in plastic "horn" or "funnel" or "flange". That's the same thing called different things. The consensus seems to be that if you are using one wax ring, the funnel doesn't add anything and actually could be a disadvantage. However, where stacking rings, it seems that our leading pros agree that you use a horn-less ring on the bottom and one with the horn on the top. That makes sure the one on the top stays in position on top of the bottom ring and doesn't slide around too much when you start compressing the toilet down. It also seems to be the consensus that you never use two rings with horns; you are asking trouble by creating the little chamber between the two horns.
Now this doesn't mean that I haven't seen some posters say that doing this is a "hack job" or "ghetto plumbing". Both characterizations can be found on here. But if Terry says it's the way to go for a DIY-er who wants to end up with a leak-free seal where the flange is below the finished floor, then I think the discussion is over.
So, to review: you can chuck that flange-extender (or return it), and go get yourself one good wax ring without a horn and one with, and go to town, placing the one without the horn on the flange below the floor level and then putting the thick one with the flange on top. I have seen it said on here that you can't get a thick one without the horn. That's actually untrue, because they are available. (Fluidmaster makes one, for example.) However, the only thick ones you will find at Lowe's or Home Depot come with the horn.
And remember, the pros put the ring (or in this case both rings) on the floor and place the toilet down vertically on top, rather than trying to stick one to the toilet first and then putting the toilet down, regardless of what the toilet installation instructions say. Just be sure that you have dry-fit the toilet first and decided where you will have to shim to make it level and rocking-free, so that you are not sliding it around on top of the wax while you are trying to decide where to shim it.