This shouldn't affect live reptile shipping, as dimensional weight has always been in play for air shipments like FedEx Priority Overnight.
The change is that they are now applying it to Ground services as well.
FedEx and UPS calculate dim weight using the same divisor, 166.
We cover dim weights, and how to calculate dim weights, at the ShipYourReptiles site as well. It almost always comes into play when shipping live reptiles, as even a 12x9x6 box that physically weighs 1 pound will actually bill at 4 pounds, as that is the dimensional weight, and FedEx charges whichever is the higher of the two.
Always use the smallest possible insulated box that you can SAFELY use to package the animals. A 12x9x6 box is going to be MUCH cheaper to ship than a 16x16x8 box that is the same actual weight!