Do I need a special permit or licence to buy your product?
No. Anyone can buy it, but if you plan on adding a stock to it and connecting it to your handgun you should be filing a Form 1 with the ATF because you are turning your handgun into a Short Barreled Rifle (SBR).
Is there any way I can avoid the ATF Short Barreled Rifle (SBR) law?
Yes. If you have a 16" (or longer) barrel for your Glock.
Can someone please explain to me the appeal of putting a rifle stock on a handgun? I seriously don't get it.