30mL is the maximum permitted.
PI 650 says Class 3, 8 and 9 must not be more than 30mL and they must satisfy the provisions of DGR 2.6 Excepted Package. If 50mL is needed as stabilizing agent, the shipper must split the infectious substance into two packages each containing 25mL of the stabilizer. Or, he should reduce the quantity of infectious substance to a quantity only necessitating 30mL or less Class 3 stabilizing agent.
DGR 2.6 stipulates that the maximum quantity allowed for inner packaging is 30mL. If you put 50mL you are not conforming with the provision of DGR 2.6.