Yes. It isn't worth it, you'd be better off buying a new P99QA. Not only do you have to buy the trigger mechanism, you have to get a new slide, because the QA striker doesn't work in the AS slide.
I went through this when I had the 4 P99 Military QA's built about 5 years ago. (These have the old style frame in OD, the ones they said they would make, but never did) I was lucky, because they only charged me the same as a brand new gun. (Only because I whined really loud
After a long battle with S&W over them promising to get them for me, Walther Germany got word of the broken promise that S&W made to me. They then had Earl of Earl's Repair Service contact me. I believe that Walther Germany sent the parts (Slide, barrel, trigger assy, etc.) to Earl. Earl then built the pistols up for me. They are all sequentially part numbered. They each have the European outlined back sight, instead of the dots. They all have the correct proof marks on them and they say they've been imported by Earl's Repair Service.