hello,
i want weight based charges with one condition

that is

if any user buys goods which are less than 6 KG, then they will see error
"Cannot checkout, you should buy Minimum goods of 6 KG"

and is user exceeds 6 Kg, then on every increasing weight they will get charged

like >>>>>

if user buys goods of 6kg, then he will have to pay $12 extra

if user buys goods of 7kg, then he will have to pay $14 extra

if user buys goods of 8kg, then he will have to pay $16 extra

if user buys goods of 9kg, then he will have to pay $18 extra
as on till 50 KG

then that amount will added in final checkout


please help me....how can i do this ??
in which version ???