The glycerol stock consists of a frozen laboratory strain of Escherichia coli (E. coli) that carries an artificial engineered plasmid consisting of 50 percent glycerol (CAS No. 56-81-5) and 50 percent E. coli bacillus. E. coli is a Gram-negative bacterium commonly found in the natural environment. In your letter, you stated the addition of glycerol keeps the E. coli alive during transportation in dry ice and these laboratory strains are not pathogenic and are solely used for non-clinical research.
[1] Please importantly note that only the relevant portion of the related is displayed above. You may refer to the said customs ruling for the entire ruling explanation.
[1]. https://rulings.cbp.gov/ruling/N320317