How to add Shipping method for commerce flat rate module?
The Rules module integrates with Drupal Commerce in order to handle this type of requirement. There are many video tutorials available that explain how to setup various shipping rules.

There is also a module called Conditional Rules that may be useful if you need to have branching logic in your rules.

In this case I think you would only need two rules:


Conditions: type of shipping == "Free shipping" AND order total > $100 Actions: shipping cost = $0
Conditions: type of shipping == "Express" AND order total > $140 Actions: shipping cost = $0


You would set the default pricing for the "Free shipping" to $4 and the for the "express" to $12. The default prices would be overridden by the Rules in those two cases.