The National Motor Freight Traffic Association (NMFTA) publishes a list of freight class designations, codes, and subclasses for commonly shipped items (https://classit.nmfta.org/). A subscription is required to view this list. You can read more about freight classes at Koho for free on our freight classes pages.
Freight class 50 is the least expensive because it refers to the freight that is easiest to transport for the carriers.
If you don't include an NMFC code on your BOL, it is very likely that your carrier will charge you a reclassification fee and charge you the cost of shipping the freight they they determine your shipment is.