To check your shipping settings, log into your Seller Central account. In the upper right of the home page there is a link for Settings, then go to Shipping Settings.
You will want to make sure you shipping model is Price Bands, you can change the model if needed from this page.
The next page will allow you to change the regions that you ship to, as well as the Service Levels. We recommend that you disable all regions besides the Continental US, and only Standard Shipping, Economy or Expedited can result in unrealistic leadtimes. By shipping to areas with PO Boxes, Alaska, Hawaii etc will require you to ship from your location to the customer since the drop shippers usually don't ship to these locations. This means your leadtimes will be off and you can be penalized by Amazon for slow ship rates. Usually the amount of sales to these regions is insignificant and not worth the potential downside.
The next step will be to set your price bands. Usually people will want to offer free shipping on products over a certain value, while still charging shipping on lower priced items. You can set your shipping rates based on price here. You can add bands and choose your own values to charge based on that price range. In this example all products with a price greater than $30.01 will receive free shipping.