The price of the item doesn’t take into account that tax exists. I did find this end point: https://api.twitch.tv/kraken/games/Darkest%20Dungeon/details for example that contains price data.
But since thats a standard Amazon ASIN B06XK192R7 I’m sure theres a Amazon MWS endpoint of use somewhere…
Generally the game being purchased SHOULD be the current game that is being played, and msg_param_title in the user notice example will match for the /details lookup