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PayPal Suspected Fraud on Zero Total Orders - Fix please?
This problem is common and often related to your tax settings. The PayPal Magento integration has troubles with the rouding differences that result from VAT calculations. We recently developed therefore an Extension for PayPal that is very easy to set up and carries even more feature. The problem you report will be gone: http://www.magentocommerce.com/magento-connect/paypal.html