What is the maximum TOTAL weight of allowed hazardous materials that a bus may carry at one time?

AAMVA CDL Manual, Section 4.2.2, p. 4-2

Even hazardous materials that are technically allowed on a bus, like ammunition or medical supplies, have hard volume limits. A driver who doesn't know the 500-pound ceiling could unknowingly let far more hazardous cargo aboard than federal guidance permits.