Ok so what you referring to ingame is the "Flame forger" Where you can design your very own flame, of course the bigger and better it is the more it will cost so once you are satisfied with what you have created ingame then you would click buy which will direct you to the forums where you'll make the final payment.

Now the flame particle is the actual texture of the flame, so in other words the picture of each particle, You don't need this item unless you want to change the picture of your flame.

Heres an example of a Flame particle texture