Hi everyone, I have a shipping table rate, example,

customer purchase $1- $10 dollar, first 500g pay $10, after that, each extra 500g have to pay $4 more.
customer purchase $11 $20 dollar, first 500g pay $7, after that, each extra 500g have to pay $3.5 more.

the table will keep building on that....

are there any module can set this up? pay or free, I try zencart table rate, but the function not enough to build it, I search a long time, but cannot find the correct one, pleae help.

Thanks a lot