There are a few reasons that could cause paypal to send back:<\/p>\n
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FAIL ERROR: 4020<\/strong><\/p>\n <\/p>\n When you attempt to verify PDT transaction data by posting a _notify-synch<\/em> command.\u00a0 However, in 9 times out of 10 the cause is due to the request containing an incorrect ‘at’ or authorisation token argument.<\/p>\n <\/p>\n The ‘authorisation token’, or ‘Identity Token’ is a big long alphanumeric string, to find yours log into your paypal account, click on ‘Profile’, then click on ‘Website Payment Preferences’, scroll down to the ‘Payment Data Transfer’ section – you will see your ‘Identity Token’ at the end of this section.<\/p>\n <\/p>\n This token must be included in your _notify-synch<\/em> command as an argument named ‘at’.<\/p>\n <\/p>\n