Hi,
Thank you for the reply. Until such time as http://forum.whmcs.com/showthread.php?t=17947 is implemented in WHMCS I am unfortunately stuck using the Offline Credit Card method. The problem however is that as I am sure you now, users rarely read what is right in front of their eyes, so even though with Offline CC there is a message that says "Your Offline Credit Card transaction will be processed soon" there is also a "Authorize Now" button, so they keep on trying to authorise the payment despite of the message, hence I end up getting 50 emails that a new offline trasaction has taken place and client wonders why he can't authorize the payment.
I get maybe 1 in 50 unsuccessful CC payments so it makes sense just to mark the offline cc paid the moment it is submitted or just remove the "Authorize Now" button on the invoice. The latter I think might cause issues with other gateways.
Thanks for reading and please let me know if you have a solution