From Will Roya who has decks printed from USPCC all of the time
"In addition, more recently USPCC has added a Thin-Crush stock to their quiver of options, and many newer cardistry decks are opting for this stock. Strictly speaking this isn't a different type of stock, but is rather is an optional process in which the Bicycle or Bee stock is crushed to make it even thinner."
Lyle Borders who is the VP of Operations for USPCC's largest custom print customer Theory 11, said, "Generally, USPC only offers two stocks (Classic / Bicycle stock, and the thicker Premium / Bee stock). There is no such stock as "crushed stock". Once you choose your stock from above, you have the option to have that stock "crushed". It is not a separate stock, it is an optional process."