GWMCrates has items in the crate that represent the particles. When you get that item in the crate, it is stored in a VirtualChest menu, and when you click on the particle in the VirtualChest menu, it removes the particle you already have selected, and selects the one you clicked.
Example:
I buy a Jester Crate (the name for the particle crate) from my buycraft store. I open the crate, and it randomly selects the fire particles. I can type /particle and click on the blaze powder (the representing item) and then groups of flame particles would appear every 10 ticks. That is the best i can do to explain.