Hi MV Company_288,

It's not depend on your order value shipping weight depends two criteria like as

1. Dead Weight/Actual Weight

2. Volumetric Weight

Note: whichever is higher calculated charged By Amazon

that means whichever is higher weight charged by Amazon Now understand the Volumetric calculation that is

LengthxBreadthxheight/5000

Your Length is : 45

Breadth is : 45

Height is: 12

Now Calculate V.W= 45x45x12/5000= 4.86 Kg

Now your volumetric Weight is More than 4.5 Kg that means Slab falls under 4-5 Kg if Volumetric weight Higher.

and actual weight is the Product Weight kindly check it accordingly on your weighing machine.

so your actual weight is : 300 Gm

So You have been charged whichever is higher that is the Volumetric Weight 5 Kg so Amazon charged 5 Kg slab for Shipping Kindly check your listing and update there .

If customer already order then raised an incident via help and told Amazon team they are able to reverse the same but you have to send the evidences of LxBxH