I would love to try this myself first so I feel confident how to change it on the other sites and for future references also. (But I might come back to your offer and of course give you FTP access).
I will attach the checkout.php page for one of the sites. I believe the script is on line 1199. Could you let me know where exactly I should best move this to?
I hope this one works. Would solve a lot of trouble for me.