Carrier Calculated shipping is only available to US and Canadian store owners. For store owners outside of the US & Canada, flat-rate shipping is recommended.
Carrier Calculated shipping is based on the total order weight, the originating zip/postal code (set in your general settings), and the destination zip code. Rates are real-time and are obtained directly through carrier's API system.
You can specify your default shipping carrier from the Settings/Shipping Settings section of your account. Only one carrier can be used at a time.
How to modify carrier calculated rates
If you need to modify the shipping rates returned by the carrier, you can do so by editing the modification settings under the Settings > Shipping Settings > Carrier Modification area of your store admin.