Stepped Shipping Costs Depending on Order total

I'm wondering if anyone has figured out how to get shipping costs to change depending on the order total. For example:
$1-15 = $3 shipping
$16-25 = $6 shipping
I know that a plugin needs to be written to do this, but has anyone done it or can anyone point me in the right direction. I have some php skills, but not a lot. I need code examples.