So what your saying is if I have a Remington 870 and I want to buy a gouge tammer postal grip for 20$ I need a 200$ tax stamp?
No, shotguns fall under different rules than firearms with rifled barrels.
If you don't know the ATF rules, and don't have a gunsmithing liscense, you really need to be cautious about what you create.
A Handgun with a forward pistol grip and no stock is classified as an AOW (Any other Weapon). It requires ATF application, and registration and a tax stamp ($25 for AOW if I remember correctly).
A shotgun with a pistol grip needs to meet overall length requirements to be classified as a shotgun still, otherewise it would be a short barreled shotgun (SBS) which requires ATF application, $200 tax stamp, etc.
to further complicate things, each state could be different in what they allow with regard to ownership of SBS, SBR, AOW, full auto and supressed class firearms.
I would undo what you did to the firearm, and remove pictures. If you charged any money, give it back, because the government wants tax money for FFL related work for a profit.
This is not meant to be means spirited, but to protect you and others from future trouble.
If you like doing this kind of work, get in touch with the BATFE and see what it takes to get a liscense and keep things above board.