How to tier shipping rates markup based on cart subtotal?

[Prime42]

I am looking for a way to markup quoted shipping rates (USPS and UPS) before they are displayed the to customer. Shopify seems to have a simple built in method for a single markup, but I am looking to have it tiered. In other words if cart subtotal is under $100, then markup/add to shipping by 50%. If $100-$500, then add 30% to shipping cost. If over $500 then shipping gets marked up 10%.

Any suggestions on how to do this would be appreciated.