Problem

Could not upload tracking information to Magento because "Invalid carrier specified."

This is caused when you ship orders using a carrier not defined in Magento.

Solution

Option 1: Add the missing carrier

To solve the issue you need to add the missing carrier to Magento. Unfortunately there is not an easy way to do this.

One solution is to add an extension like this one: http://www.xtento.com/magento-extensions/magento-custom-generic-carrier-trackers-extension.html

• Once enabled, in Extensiv Integration Manager go the magento settings and enable the "Custom Carrier Trackers Extension" option. Then enter the carrier names as they appear on the shipping in Integration Manager and which Custom Carrier they associate with in Magento.

Another solution is to ask a developer to add new Carrier Models to your Magento site for the needed carriers: http://magento.stackexchange.com/a/18502/15231

Option 2: Use a different carrier that is already defined in Magento

For standard carriers like UPS, and FedEx, make sure that your WMS is providing consistent carrier names that match those in Magento.