The suppressor requires one $200 tax stamp. The short barreled rifle also requires a $200 tax stamp.
If you're talking AR15, though, the rifle IS the lower, so your lower will be registered as an SBR and you can swap out different caliber and length short-barreled uppers on it. The lower is what requires the tax stamp. The uppers do not. Which is why you can buy short barreled uppers online with no FFL, no tax stamp, etc. You could theoretically put one on either an SBR'ed AR15 lower or on a pistol lower.
If the suppressor is integral to the upper, that changes things a bit. Then the upper requires the tax stamp as well, but only because it IS the suppressor. As long as the suppressor is detachable, you'll only need a tax stamp for the suppressor.
Here's the breakdown:
suppressor - needs a tax stamp
upper - just parts, does not require a tax stamp, regardless of barrel length
upper with integral suppessor - requires a tax stamp
lower - requires a tax stamp before you can put a short barrel upper on it
I hope that answers your questions. If you want to build your own suppressor, you can do that on a Form 1. If you want to buy one, you'll do it on a Form 4 from a dealer. You can build your own SBR by registering your current lower on a Form 1, and you can then put any short barrel upper on it you want, including a 300 Whisper upper than you buy or build.
Aaron