I believe it is a one-time payment. Same with sound suppressors. The ideal home defense weapon would be a 16inch barrel (counting suppressor body welded on) AR platform in 45 ACP caliber ... IMHO. You can build one for around $1400, with three twenty-five round mags.
They want $200 bucks for each Class III item though don’t they? One for your full-auto receiver, one for your suppressor and one for each of anything else on the list. And a brand new registration and another $200 if you alter anything such as putting different guts in your suppressor. I could be wrong about that.