Aitcheckout extension package usually contains readme.txt file with the following contents:
COMPATIBILITY WITH SAGEPAY SUITE CE (PRO)
For the instructions, please refer to the help guide named "Making the extension compatible with Sage Pay Suite CE (Pro)" available at Dosc & Help tab on the extension page.
Here's also a direct URL for downloading the help guide: http://www.aitoc.com/en/aitdownloadablefiles/download/aitfile/aitfile_id/700/. Once the archive is downloaded, please refer to the Readme.txt contained within the archive.
There are known problems when calling quote methods if this module is enabled.
If the integration used is SERVER:
open up app/code/local/Ebizmarts/SagePaySuite/Model/SagePayServer.php and comment $quoteObj->setTotalsCollectedFlag(false)->collectTotals();
open up app/code/local/Ebizmarts/SagePaySuite/Model/Api/Payment.php
$this->_getQuote()->reserveOrderId()->save(); to $this->_getQuote()->reserveOrderId();
In order to solve the problem where attributes are not saved you should do this.
Open app/code/local/Amasty/Customerattr/Model/Rewrite/Customer.php change line 49 to
if (!$customerAttributes && ('checkout' == Mage::app()->getRequest()->getModuleName() || 'sgps' == Mage::app()->getRequest()->getModuleName()))
Open up app/design/frontend/base/default/layout/sagepaysuite.xml and add <firecheckout_index_index> node with the same contents that node <checkout_onepage_index> has. Clear magentos cache and try again.
If you get this error message: This customer email already exists, the guys from OSC support added a double check for this problem, make sure you are running the latest release of their module. The fix is present in 4.* series and is turned on dependant of registration mode set from System > Configuration > Sales > OneStepCheckout . You can force this to be always on if you edit app/code/local/Idev/OneStepCheckout/Helper/Checkout.php and set the line 132 to return true all the time.Payment has failed, please reload checkout page and try again. Your card has not been charged. (ONLY ON IE BROWSERS)
Add these two statements
Common Problems >