Due to a bug in the latest version of Magento the delivery charge is not sent to Paypal so customers will be charged the grand total minus the correct delivery charge.
The issue is documented here http://www.magentocommerce.com/bug-tracking/issue?issue=8467
If you’re not familiar with patch, you can download these patched files, magento1.4_paypal_patch.tar, extract and upload to your server.
