The Mattes correction pad pricing includes a full set of shims.
I started answering about Mattes because that’s the question you asked, but I most often recommend ThinLine Trifecta 1/2 pads. They have a quilted underside with ThinLine panels sewn onto the top side. Cut to allow for wither relief, easy to wash, and easy to add shims to. About $138. I like them because ThinLine is an excellent shock absorbing material, for the horse’s back AND for the rider’s back. And, without shims, they don’t alter your saddle fit.
With shims added, you can slightly lift the front of the saddle (or fill slight hollows behind the shoulders), fill in a slight bridge gap in the center, or slightly lift the back of the saddle. I’m emphasizing slight because the ThinLine material is, well, thin… so it’s best used for minor corrections. And, frankly, if you need major corrections you really need a different saddle instead of a half pad. If your saddle is a pretty generic shape that tends to fit a lot of different back types without much issue, the ThinLine Trifecta could be a better use of your $$ than a Mattes.
But if your saddle has a particularly wide tree, very little wither clearance, or a very straight tree/long panel, the thickness of the sheepskin Mattes will help it adapt to more horses than the ThinLine (because the thickness will fill in deeper gaps, and lift the entire saddle up higher away from the back, which can be key in dealing with fit issues like high withers or a particularly curvy back under a flat tree/panels).
So, to echo what others have said… Which pad you need depends on which pad you need!
I have four horses and ride many others using close contact, dressage, treeless dressage, and endurance saddles, so my half pad collection is pretty extensive… ThinLine Trifecta, ThinLine Comfort Saddle Fit (sheepskin underside with shim pockets), Fleeceworks Balance with front shims, Fleeceworks Sheepskin FXK, Cavallo All Purpose, and a Toklat Matrix Sheepskin with Extreme Pro-Impact inserts. What combination I use depends on the horse and saddle.
Sorry it’s not easier to just say, “buy this one,” but the text in red is the best advice I can give without seeing your saddle and the horses you’re putting it on. 