So you tried out or tested a shipping module / extension in Magento that didn't quite work out to be what you wanted. You then went and uninstall that extension. Next time that you are trying to view and process orders, you are likely to end up with an error. If you haven't already figured out how to view the full / complete error. Follow these steps
1. Note down the error you are getting in the admin. It's a long number like: 942421237527
2. Login / FTP to your site or File Manager for your site and have a look in the base of the Magento directory and you will find the var directory.
3. Go in to var/report and you will see a file called 942421237527 - view that file and you will see something like:
a:5:{i:0;s:44:"Invalid model for shipping method: flatrate2";i:1;s:6824:"#0 /home/user/domain.com.au/app/code/core/Mage/Shipping/Model/Config.php(104): Mage::exception(’Mage_Shipping’, ‘Invalid model f...’)
#1 /home/user/domain.com.au/app/code/core/Mage/Shipping/Model/Config.php(44): Mage_Shipping_Model_Config->_getCarrier(’flatrate2’, Array, NULL)
#2 /home/user/domain.com.au/app/code/core/Mage/AmazonPayments/Block/Adminhtml/Shipping/Methods.php(64): Mage_Shipping_Model_Config->getActiveCarriers(NULL)
#3 /home/user/domain.com.au/app/code/core/Mage/AmazonPayments/Block/Adminhtml/Shipping/Methods.php(37): Mage_AmazonPayments_Block_Adminhtml_Shipping_Methods->getShippingMethods()
#4 /home/user/domain.com.au/app/code/core/Mage/Adminhtml/Block/System/Config/Form/Field.php(91): Mage_AmazonPayments_Block_Adminhtml_Shipping_Methods->_getElementHtml(Object(Varien_Data_Form_Element_Select))
#5 /home/user/domain.com.au/lib/Varien/Data/Form/Element/Abstract.php(202): Mage_Adminhtml_Block_System_Config_Form_Field->render(Object(Varien_Data_Form_Element_Select))
#6 /home/user/domain.com.au/lib/Varien/Data/Form/Element/Abstract.php(212): Varien_Data_Form_Element_Abstract->getHtml()
4. Notice where I have bolded "flatrate2" above, this will change depending on the shipping module you used or uninstalled. Common ones are australiapost if you uninstalled the Fontis extension or freeoptionalshipping if you uninstalled Auction_Matrxrate extension.
**MAKE BACKUPS OF YOUR DATABASE BEFORE PROCEEDING**
5. Copy the shipping method (in my case it is flatrate2) and login to PHPMyAdmin on your control panel. Look at the tables on the left and you will see core_config_data. We need to view that table and delete all the rows that contain the instance of your equivalent of my flatrate2
6. After this is complete, refresh your cache in Magento and try again. I have occasionally seen the need to restart Mysql also because of it's own cache though this shouldn't ever be the case.