Se o add_prazo não estiver preenchido nas configs do módulo, teremos algo como:

Warning: A non-numeric value encountered in ...//app/code/community/PedroTeixeira/Correios/Model/Carrier/CorreiosMethod.php on line 386

#0 ...//app/code/community/PedroTeixeira/Correios/Model/Carrier/CorreiosMethod.php(386): mageCoreErrorHandler(2, 'A non-numeric v...', '/Users/martins/...', 386, Array)
#1 ...//app/code/community/PedroTeixeira/Correios/Model/Carrier/CorreiosMethod.php(196): PedroTeixeira_Correios_Model_Carrier_CorreiosMethod->_appendShippingReturn(Object(SimpleXMLElement))
#2 ...//app/code/community/PedroTeixeira/Correios/Model/Carrier/CorreiosMethod.php(121): PedroTeixeira_Correios_Model_Carrier_CorreiosMethod->_getQuotes()
#3 ...//app/code/core/Mage/Shipping/Model/Shipping.php(209): PedroTeixeira_Correios_Model_Carrier_CorreiosMethod->collectRates(Object(Mage_Shipping_Model_Rate_Request))
#4 ...//app/code/core/Mage/Shipping/Model/Shipping.php(129): Mage_Shipping_Model_Shipping->collectCarrierRates('pedroteixeira_c...', Object(Mage_Shipping_Model_Rate_Request))
#5 ...//app/code/core/Mage/Sales/Model/Quote/Address.php(944): Mage_Shipping_Model_Shipping->collectRates(Object(Mage_Shipping_Model_Rate_Request))
#6 ...//app/code/core/Mage/Sales/Model/Quote/Address.php(875): Mage_Sales_Model_Quote_Address->requestShippingRates()
#7 ...//app/code/core/Mage/Sales/Model/Quote/Address/Total/Shipping.php(155): Mage_Sales_Model_Quote_Address->collectShippingRates()
#8 ...//app/code/core/Mage/Sales/Model/Quote/Address.php(1013): Mage_Sales_Model_Quote_Address_Total_Shipping->collect(Object(Mage_Sales_Model_Quote_Address))
#9 ...//app/code/core/Mage/Sales/Model/Quote.php(1331): Mage_Sales_Model_Quote_Address->collectTotals()
#10 ...//app/code/core/Mage/Checkout/Model/Cart.php(461): Mage_Sales_Model_Quote->collectTotals()
#11 ...//app/code/core/Mage/Checkout/controllers/CartController.php(162): Mage_Checkout_Model_Cart->save()
#12 ...//app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Checkout_CartController->indexAction()
#13 ...//app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(254): Mage_Core_Controller_Varien_Action->dispatch('index')
#14 ...//app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#15 ...//app/code/core/Mage/Core/Model/App.php(365): Mage_Core_Controller_Varien_Front->dispatch()
#16 ...//app/Mage.php(686): Mage_Core_Model_App->run(Array)
#17 ...//index.php(83): Mage::run('', 'store')
#18 {main}